From 1968 up, you can pretty much interchange any parts with any years . . . however, if you change one part, a lot of times you might have to change two or three others to make everything fit/work correctly. For instance, in the interior, if you buy new door pannels, they will fit any 68 or later Beetle, but the arm rest changed somewhere along the line. We installed an earlier driver's door on Justin's 74 super many years ago and everything swapped over from the 74 door to the older door, but we changed the arm rest to the earlier style rather than try to install the plastic pieces the new style arm rest screwed in to. The only problem was we had the new style arm rest on the other door . . . so we cut out the metal tap that holds the old style arm rest strap from another old door and rivieted it on the 74 door so we could use old style arm rest on both sides.
As for complete engines, just about anything from 67 and up will interchange, but you can't bolt the intake off a 67 on a 74 engine 'cause one is a single port and one is a duel port. You can put the engine from a 79 fuel injected Super Beelte convertible in your 74, but you will have to convert it to a carb and install an electric fuel pump . . . and I don't think the head tin will fit, so you'll have to use the non-fuel injected tin . . .
Lots of little things changed from year to year so like Ret.Bugtech and cetdubtech said . . . it's hard to say & there is no quick or simple answer to that question. It pretty much has to be a part by part question.
