Stock pipe ID is about 1¼", but that's way too small.  Some aftermarket pipes are up to 50mm ID, but that's a little too big unless you're wanting to go for a really wild tune and overbore.

Keeping to 650cc and not wanting to rev much higher, but porting, sorting cam and carb, a 1¾" header would be about ideal.   Anything from 40-45mm would be ok.

I used 1.75" OD pipe, with 1.6" ID.  This was with a performance cam and performance carb kit + a great muffler.

With those basic mod's this pipe works great.  I had intended to try a 1.5" ID with transition to 1.6" ID and then 1.75" ID before the muffler, but have yet to get it down.