I'm reckoning Outboard gearbox/output bearings are all about thrust, they take no load across them, unless you've got a buggered prop, the vibration from one of them shags pretty much everything.
the output shaft is pretty thin, certainly less than a stub axle, it's also not very long, as well as being pretty much under the engine.
Cooling would be a complete re-plumb, as the engine sucks water from the bottom of the leg, circulates it round the engine and spits it out through the middle of the prop. how nicely you could convert it to normal pipes i don't know.
most modern outboards are 4stroke EFI, so with some sort of closed loop control speed control should be do-able.
I've looked and an 80HP (Yamaha) outboard weighs 170 Kg, 680Kg for four corners.
the leg probably isn't designed for any sort of force apart from it's traditional push, neither is the mounting bracket.
putting the engine on a new drop shaft and gearbox might work, but that must be mucho $$$$.
I'm guessing this is purely as a rock crawler type thing? the speed variation with one gear is going to be pretty low. you don't get a clutch either.
Luke