165R15 is the modern size for the TD/TF wheels. Steel belted radials are a must-have safety item with today's driving speeds. The TD wheels are not air-tight due to those large rivets holding the rim and center section together, so you need to buy inner tubes also.
The 165R15 tire is also used on the classic VW Beetle, so your favorite tire store may have them listed for that car if they draw a blank on MG data. Since they're an odd size by today's standards, you may have a short wait for shipping from a central warehouse.
I wouldn't spend big $$ for some sort of high performance tire for your TD. Unless you're into vintage racing, our TDs won't put much wear on the tread. The tires will most likely "age out" before you chew up the treads by any significant amount.