geeze
if a YZ motor is fitted to a Honda (some might argue a huge improvement) and both the components are pre 75 for example its OK, what isnt OK is to take a Unitrack KX and turn it into a twin shock and race in the EVO class, the equipment can be mix and match but you cant cut up later stuff to fit another class.
Read the rules, if it says you can only use a round slide away you go, if it allows flat slide then its noted there in, if you could get a DG swingarm in 74 (or a suitably correct replica) for your 74 Honda happy trails its OK - but if you have a DG swingarm that was only available in 78 on that bike its not OK. Its really clear and really simple
OEM has been in the rule book for years and is there after newer bikes were cut to fit mainly in the EVO class.
As for the bars and levers................