Ancient Greek,
Most of your post is correct but your first sentence is a bit sweeping. Yes, CS-23 / FAR 23 does contain rules for a very broad range of aeroplanes but as I know from recent experience (I've spent the last two years on the GameBird team), if you build an an aeroplane with fixed gear, conventional instruments, manual controls, unpowered brakes, etc. most of it can be marked not applicable. The minute you start adding primary flight displays, powered brakes, electronic engine control, etc. you get into the difficult certification territory. A new aeroplane can be done for single figures of millions of US dollars if one is sensible about it but because of the sales volumes it takes years to get it back and modern capitalists don't like that.