I read this topic with interest, as I am contmplating mods for my TY175.  This is due to low support for twinshock classes where I ride, and not non-stop rules. 
So basically I compete on a course set for modern bikes, requiring stop turn, balance, squirt throttle riding - quite hard with old technology, just want it to perform more user friendly. 
So where I am headed, to get more mid range punch by the way of mods, so that I can ride these courses comfortably. 
I have even modded my Rev3 Beta 270 for better control, so this is bound to happen along the way to any bike. 
I agree with keeping to the original frame, brakes, suspension etc. 
Here is one bike that defies the twinshock catagory - Mick Andrews 1973 Yamaha TY250 OW10, its a monoshock!   A Rare bike indeed, and not likely to turn up at any ride soon.  Scroll down link below to view. 
http://translate.google.com/translate?hl=e...l%3Den%26sa%3DG