I've been using a modified GX35 for 3 or 4 years, it turns 20x8 master at around 7100RPM in a 104" glider tug/lolly bomber and has plenty of power.
I used a GX31 piston and machined the top down to give it about 10.5-1 compression, filled and reshaped the inlet port and unshrouded the valves, removed the compression release in the cam, gutted a GX25 muffler, installed a RCXL ignition and fitted a walbro carb from Dave's Discount motors that has the next size up bore, 10.5 mm I think.
A great little project, but not cheap!