Charles: As an owner of a 782 that has had three (3) different engines in it, I think you can physically mount just about any kind of engine you want to in a 782. I've had a Briggs & Statton 16 HP, the original Kohler KT 17 Series I, and the current Kohler 18M that I had to buy an entire tractor to obtain. Dennis Frisk can speak to the power and problems ($$$) of owning an Onan. What I noticed tearing down a 1250 and comparing its mounting to the 782 engine bay is that I could even mount Quiet Line engines in the 782, although I would probably want at least a well-tuned 12 HP unit to power the hydralics. Depending on how I much I like the way my 149 runs, I may even go back with the 12 HP unit out of the 1250 if anything goes wrong with the Magnum motor. I probably don't need all that power just to mow the grass.
The biggest issue with the engine replacement is getting the drive train sorted out. It seems the B&S was NEVER attached to the drive shaft like it should have been when I bought it. It had let loose at least once and the steering column suffered damage from which I don't think it will ever recover. (I've repaired it, but it still "sticks.") It was only because of the sharp eyes of Matt Gonitzky that I discovered that the Sears Magnum I was using did not have the proper interface to the drive shaft. I had to locate and install a drive cup that would properly accept the Cub Cadet mounting flange. It turned out that the bearing on the other end of the shaft was missing and I didn't know it. When the whole thing is properly assembled, it works pretty well. You just need to make sure you have ALL the proper parts and only modify what needs to be modified for your application.
I think the Onan motor that was fitted to the 982 was 20 HP. If the flywheel on your motor has the same configuration, you should be able to convert it for use on a Cub Cadet, otherwise, you're going to have to build a custom setup and you need to know what you're doing when you do that. There is a FAQ about installing a U-Joint type coupling at the rear of the tractor, and I think at least one forum member has used a U-Joint on the front so that he could mount a B&S V-Twin OHV motor.
Again, you can mount just about anything in a 782 if you have the desire and the skill, or know someone who does.