Flyer,
Here is a thought. If the GX31 piston is taller above the wrist pin then the GX35, the rod must be longer then the GX35. Due to the makers use of standard needle bearings, the longer rod would have the same bearings. So, put a GX35 rod into a GX 31 and get higher compression. If this is true, you may have to notch the piston slightly for valve clearence. Who would know?

Dave