Thomas46
Check with Bob at RC Boat World.He developed all the mods for the Honda GX31 and GX35 and helps me all the time.His GX31 spins a woodpusher 20X8 to 7400 rpm.
I called him about the cams because hes modifiying a GX35 for my giant Telemaster.

The GX22 and GX31 are push rod engines.

The GX25 and GX35 are OHC.

The model designation doesn't change by country.

The bore size in the GX31 and GX35 are the same so the pistons will interchange, modifying the compression height of the GX31 and putting it in the GX35 can yeild compression ratios from 10:1 to 12.5:1 with no valve interferance on stock cams.

The GX25 and GX35 cams do not directly interchange as the hub length of the GX25 is longer than the space in the GX35 but can be modified to fit.

They are also timed differently as noted above, a GX25 cam in a GX35 can advance or retard the lobe centerline 15 crankshaft degrees.

15 degrees is normally considered too far retarded but on the stock engine the timing is so short it won't hurt especially if combined with a CR increase.
Retarding the cam moves the torque peak UP the rpm range.

Mono

try the advance at different settings, on Bobs high rpm boat engines its running @ 22 btdc for max power.