Hi,

I'm not too sure on this, but I would imagine that the Pro .61 may have more aggressive porting than the GP .61, simply because it is a sportier engine. If this is the case, then modifying the Pro .61 to take the Gp .61 piston/liner may just leave you with a slightly odd-ball engine with very few bnefits - maybe even less power (but possibly more lower down in the rev range???). Just a thought.

Regards
Warren