My 2c opinion re the pre '75 class... go for the simplest option: As a starting point anything that is as per the original manufacturer spec is legal. That way all unmodified bikes are legal, nothing for the happy vintage racer to do in order to compete.
If there is concern about competition across brands perhaps allow competitors so inclined to modify their machine to match a nominated spec. For example a '73 maico (or whatever) could be altered to '74 specs (or whatever).
Those two basic ideas allow both easy access to the class and room for the tinkerers to tinker. You know, same as it was back in the day. :-)