If the engine has not been run or even hand turned in 45 years, there's a good chance the rings have detensioned themselves and perhaps even left impressions where they have lain all this time. Before pulling anything apart, however, I would find someone with a boroscope and have a look through the sparking plug holes at the piston tops and the cylinder walls. Who knows? Disassembly of an XK series engine's upper end is not for the faint of heart or the light of experience.
I've seen engines go to hades in a hanbasket in only a few years of storage. On the other hand, I've seen forty year stored motors start right up, not smoke, and go on to lead a fine second life. Pull the plugs. DON'T turn the engine over yet. Have that look inside. Put a squirt or two of light oil in each hole. Let the thing sit for an hour. Now turn the crankshaft and look at the areas you couldn't see at first. If they look acceptable, you may have a decent lump there.
If you really want to be sure of this motor, next flip it over and remove the sump. This will allow you to remove the caps of the main and rod bearings for inspection. Use Plastigage to determine if the crankshaft and bearings have the correct clearances. If this all checks out, replace any doubtful looking gaskets and seals and let 'er rip! (Well, install it and crank it over til you get oil pressure with the plugs out, THEN let 'er rip!)
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon