I would keep it simple. Use a lower gear of a small outboard motor 2-3 hp range, its very simple to attach a shaft and a motor to it. The lower gear can be integrated to the the mast. The shaft is very thin on those, can go inside the mast. Small outboards have usually roughly 1/2 gearing, that is actually required to make a small gas engine work. Torgue is also an issue on those.
Actually think this would work well on an e foil, no more leaks and all electric parts on top of water.
Have been doing some testing on gas engine on surfboard and have noted that also a 3.5 hp ( 75cc 2 stroke ) outboard is sensitive to lack of torque. My tohatsu 3.5 really struggle with the Yamaha clone propeller with 8 pitch , same problem as the 65161 type motors also have. The problem is the diameter, not the pitch as such.
The critical moment would be to get it to fly, you would need low pitch propeller otherwise engine might stall. My gut feeling is that a Honda GX 50 might do the job with the right propeller and gearing, not sure a gx35 would be enough.