There is a reason the TC states engines built after a certain date can’t be installed in certified aircraft.
Single ignition: "No aircraft of these models shall be eligible for original certification with single ignition engines after August 1, 1941. In addition, no aircraft of these models shall be eligible for recertification with single ignition engines unless such aircraft were either previously certificated with single ignition engines or were originally certificated prior to August 1, 1941." (A-696)
The A-80 is an oddball engine that you don’t want to use any parts from.
I have a Continental manual from 1948 that (on page 4) states "NOTE: Models A75 and A80 are no longer in production." They had moved on to the C85.
My A&P, a very experienced guy, is pretty unimpressed at the stated horsepower increases (for instance the lines on performance charts for the A75 and A80 lay right on top each other in the manual). A50, A65, and A75s engines consume the same fuel at the same horsepower. The latter two just have higher top ends.
A-50 is different from A-65 and A-75. Not sure off the top of my head, but there are differences.
The cases, -1, -8, -9 for instance, apparently have lots of differences. As time went by the case was modified so more items initially integral to the case became separate bolt on parts. (IE an -1 cases, for example, had only a single magneto mount. -8 has no provision for a starter. -9 cases do. But because of the mounting pad, the magneto mounts were separate parts and required gaskets and plugs and the change limited the type of mag that could be used.)
"(1.) General Description (a) All the engine models are identical in general construction with the differences in the power rating of the engine, maximum R.P.M., compression ratio, number of piston rings..." (page 4)
It goes through details of the improvements. It also says that where improvements were made the previous parts are now obsolete, no longer being produced, and details how the improved parts can be substituted. (IE: the older pistons had four rings, newer ones three. You can replace a set of flat pistons with the improved domed ones, etc.)
The A-65 can be converted to an A-75, and rumor has it that many later A-65s had all the A-75 parts when they left the factory.
A65 to A75: Replacing the carburetor venturi and jets allows the engine to ingest more air and spin faster, that and putting a slightly smaller lighter propeller creates a practical A75, even if legally its still an A65.