Have you thought about the gearbox from something like what is commonly called a bush hog rotary mower. They are common in agriculture. Larger mowers easily put 100 HP thru them. Sorry, I have no idea if they come in different drive ratios. They are classified in HP ranges.
Outboard motors have a connection on the bottom for the drive gear and whatnot. There is also a flywheel on the top of the engine. If, as someone else suggested, you are building a helicopter I would think an outboard engine would be an excellent choice for a powerplant. Great power to weight ratios and you could rig the propeller to the top of the engine with fair ease.
The homebuilt helicopters use a 5 or 6 groove pulley bolted to the flywheel of a two-stroke outboard. It is usually a Johnson or Evinrude 99.6 cid cross-charged(as opposed to loop-charged) V-4(85 to 140 hp)... there are manifold plates made to bolt to the bottom of the powerhead to accommodate the cooling and exhaust systems. You will need a closed-cooling system for flight, obviously(water pump & radiator), and a tuned exhaust (stinger, expansion tube, and muffler).
The bottom of an outboard is not a good place to drive much more than the water pump in this configuration. It is just a splined crankshaft, and would require some kind of full-bearing gearbox to be useful. Not very practical when the cooling and exhaust is confined to the same area.
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.