982's with the 20hp Onan are noticeably more powerful than the 17hp/18hp Kohler twins. The 20hp Onan will put a hurt on a 20hp Kohler Magnum as well, just not as much of one. I have run 982's with the Onan, 20hp 782's, 18hp 782's, 17hp 782's, 1872's, and 2072's, as well as a 782 with a 18hp Honda repower. I would prefer the 20hp Kohler, only because they are so much cheaper to keep running, BUT the Onan is the one with the big torque!!!
That said, anything Kohler 18hp Mag or larger will be just fine in a 982 frame.
Personally, I would never use a 9 for parts for a 7, but rather the opposite, and use good 782 parts to make a great 982. The extra 5-1/2" of frame and bigger tires and front axle are welcome improvements.