I have found this and other threads on Honda engines facinating.
I have converted a new GX-35 recently and it's going very well with a few changes.
It's turning a Master Airscrews 18 x 10 at 7100 Rpm and a 20 x 10 at 6600 Rpm. Great touque.
The changes are,
GX-31 piston fitted with .025" machined of the top this gives 10:1 compression (measured). This give about .020" squish. The original piston runs .050" down the bore and has a dish.
WKY 194, 10.5mm Walbro carby (adjustable mixtures) on drilled out manifold, no fuel problems.
Inlet port filled around valve guide area and ground to smoother profile.
Combustion chamber cleaned up, mainly unshrouded the valves, it's very bad on the standard engine.
GX-25 cam fitted with decompressor removed, the cam profile and gear is the same as the 35 but the lobe centre line is in a different position, same top end performance with much smoother idle, about 1300 Rpm.
RCXL ignition set at 32Deg BTDC.
GX-25 muffler with 12.5 outlet and baffle removed. From my testing it's about 300 Rpm better than an open pipe.
Most of these mods have been mentioned in this forum.
I hope someone finds this useful.