Let's think about the condition of the powerplant first. Your compression numbers aren't all that bad, yet you indicate the engine burns oil. Have you changed the oil? A compression test is really only a good way to see if there are any gross problems in the cylinders. (holes in the pistons, burnt valves,etc.) In order to know where your losses really are, you need a cylinder leakage test.
If the previous owner was lax in his maintenance you probably do indeed have deteriorated lifters and the cam lobes may also be suffering. The rough running you describe may be nothing more than tuning issues. I suspect, though, that you are determined to modify this engine for better performance.
There's an old saying in the performance game. "Speed costs money. How fast did you want to go?" I wish performance was something we could simply order up in the form of one or two parts and bolt on. If wishes were horses we'd all have five or six hundred of them under our bonnets. I don't wish to discourage you. There are items which can give reasonable gains for fair money. But, you need to set reasonable goals and expectations for what is essentially a nearly sixty year old design.
Your question concerning the best bang for the buck is a classic. The easy answer is a freshly built total overhaul in stock form. I have the impression that easy answer is dissapointing to you. It would be to me as well, but the fact is that dollar for dollar that's where the greatest gains in an old engine are to be had for the least expenditure. But there is indeed more.
Next would be to take that freshly rebuilt engine and add a supercharger kit to it. WHOA! that's about four grand when you take into account the heat shield and the larger exhaust to handle the flow and the boost gauge, etc. And then there's the installation time and the tuning to take advantage of the money spent. It's a pile of bucks, but in terms of bucks spent versus power gained, it's the best bargain out there. So... do we need to reframe the question?
Maybe, maybe not. If you can afford the complete overhaul down to the last detail (We generally wind up charging about six thousand for this, but do EVERYTHING you can imagine and warranty the unit for two years of unlimited use). Perhaps you can accomplish the work yourself. That will save you a bunch. But there are still the parts and the machining and balancing to consider, and you do not want to skimp when you plan this phase. Well, if you can make this outlay of cash and then pop for the blower, that's definitely what I'd do.
If, on the other hand, the real question is, "What can I do that will brighten up the performance of this car without a great deal of extra dollars above the engine work, I'd say, "Make it breathe". You've already got a start with that Mikuni. Next is some mild port work on the head, followed by a header port matched for good flow, and a larger diameter exhaust system. A better cam will also help in the breath department.
The subject of camshaft details is too long and too full of controversy to even start to discuss here, but there are a number of mild upgrades from Kent, APT, Moss (Crane, I believe) and others. I would not advise going too far here until I knew exactly what I wanted in the way of performance and what I was willing to sacrifice in the way of tractability. (Low speed drivability) It goes on...
How about higher ratio roller rockers? How about increasing the compression? As you get into it, you'll find that each little increase over what you've already done to get to a certain point costs exponentially more and subjects the engine to just that much more stress. So, you need to ask yourself how much more you want and how much more you are willing to spend.
The aluminum head is nice. In fact, we rarely rebuild a street MGA or B without adding this part. This is actually more for reliability and cooling than for performance. You will find many who say that the porting is not optimal in these heads for top performance. We've had very good experience using them along with a mild performance camshaft such as the Kent stage one or two. We usually shave the head a bit and deck the block to raise compression. Be aware that we operate at five thousand feet of altitude here!
Here is the engine I am currently putting together to install in my MGA: 18V series MGB block and crankshaft, .060 oversize 8.75:1 pistons, APT VP10 camshaft w/matched lifters and springs, Isky pushrods. vernier adjustable cam sprocket set, 1.6:1 roller tip rockers, non crossflow aluminum cylinder head. oversize "gas flowed" valves, reworked distributor with electronic ignition, high volume oil pump. Weber DCOE 45 carburetter, Maniflow header which will be ceramic coated, and an improved crankshaft damper. Those are the major improvements in the parts area.
Next, the block will be decked and aligned in all dimensions, the crankshaft will be ground and polished to a fair-thee-well, the rods will be rebuilt and aligned and balanced end for end so they match. The pistons will be match balanced. The head will be milled to achieve as close to 10 to 1 ratio as possible, the block will be relieved to allow for the oversized valves and to "unpocket" flow around them. The combustion chambers will be equalized for volume. The ports will be mildly reworked in the head and matched to the manifolds, the flywheel will be lightened (This will improve response, but not increase horsepower), every reciprocating or rotating part will be balanced to a high degree.
There's more, but I can't remember it all right now! What will all this give me? I'm hoping for about 127 horsepower. Big deal, eh? Well, I wanted to build a lively unsupercharged engine as an exercize to show customers what is possible in an engine that can be tolerated on the street. I bet you can begin to see how much more easily a supercharger could be applied to a stock engine, and I haven't even begun to describe the hours of careful assembly to assure that everything clears and that I'm not building a two minute grenade.
Obviously, I'm not writing this post just to respond to your question. Lots of folks speculate as to what they should do for some more power. The fact is, there is little that is meaningful that you can do without considering the entire system, end to end. Sure, a header and better induction is a relatively inexpensive way to make an improvement, and I think that's a fine way to go, but a real leap requires consideration of the entire package.
If power is more important to you than "MGness", consider adapting any of a large number of high performance twin cam four valve per cylinder Japanese engines and gearboxes. The modern engineering and reliability are incomparable. Or... think about that supercharger...Please let me hear from you about your goals and desires. There are lots of options and opinions. Above all, enjoy that car.
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon