i would hate to go from a 67mm piston down to 65 fearing even less power ... but this is why it's a hard find.
but throw enough money at anything it's no longer a problem I guess.

that said I've seen genuine 167fmm (in gray) for under 500$ sold from within the US, I'll try to scare up the link.

zorge is right for 750 its not really solving the problem.
for that I'd invest the time and a few more dollars and see to fit one of these with a similar engine mount pattern

http://www.ebay.com/itm/262029549770

4 valve ohc engines, likely has twice the power

hey zorge what this engine a clone of?
some Honda I think...