[sorner]

I have the 982's cousin, the 1912. It had a bad Onan in it when I got it and I swapped it out with a Magnum M18, and later an M20. That tractor is a beast... that said, if you have the wiring harness from an 1872 you're in good shape. The key switch is pretty much the same, I'm not sure how to tell them apart visually, but internally, the wire that would go to the coil(s) for the Onan would have battery 12v at the wire to the coil while the key is on and not have 12v when the key is off, while the Magnum should be "open" with the key on, and grounded with the key off. You want to make sure you have the right one, because applying 12v to a coil the magnum will ruin it. My 1912 rather than replacing the wiring harness, I used a relay to get around the wiring difference so the 12v controls the relay and grounds/opens the coil circuit, there are posts about that here I think.

Yes the driveshaft should work.

I have spacers under my magnum, about 1/2" if I remember.