You mention that it's all original so that must mean something to you yet want to modify it with bolt on performance parts? Not that it matters.... just curious. Anyway, there is most every type of modification for the 273 that there is for any other V-8. There are a few differences that make some items harder to find or more expensive but most are or have been made. The heads for the 64-65 have a different intake bolt size and angle so intake manifolds are a bit hard to find. Headers are a very tight fit in these cars so only a few are made and fairly expensive. Otherwise whatever you can buy for another car is typically available for an early A with a 273. The 273, 318, 340 and 360 are all the same engine family so there is a fair amount of parts overlap. The 273 four barrel is a pretty decent engine to begin with so if it is healthy you may not want to change a lot.
Are you talking about things like igniton upgrades or carburetor, or........... Or are you talkig turbo, blower, nitrous......?