Guys here is what I know about the pistons etc. For starters here are the bores and strokes for these engines:

GX25 33mm bore 26mm stroke
GX31 39mm bore 26mm stroke
GX35 39mm bore 30mm stroke

The 31 piston needs to be fly cut on top to fit the GX35. The area above the top ring is taller or thicker than on the 31 piston. What you will do is increase the compression ratio. Stock is somewhere in the 8's as I recall. There is a formula I have somewhere but suffice it to say that you can get 10.5:1 safely with this mod and the engine will run fine. The issue is a coating of some sort on the pistons. You can break through it with a fly cutter, however no one has been able to say how if at all eliminating this coating will affect piston longevity. I talked to and got this information from a guy on the East coast that builds these engines for boats. He claims to get in excess of 10,000 RPM from these engines. He also produces cams for them. When I talked to him last I decided that for my use the GX31 I have would not gain much by swapping cams. Maybe with the piston mod on a GX35 and adding a cam you might be on to something. Now if I could only find a GX35. They have them on Ebay now for $225.00. By the way when you do these engines remove the compression release crap from the cam. I ran mine with a CH ignition and it would idle so slow the compression realease caused issues. Removed it and it runs great.