Hello, Rospaw;
Welcome! I hope your Max brings you great times.
What you decide to use for power is really up to you as to what you want to do with it and how much you'd like to spend. The Max IV is a good sized machine, so personally I wouldn't use anything under 18 horses, and the bigger the better. I won't even suggest a two-stroke because of the grief I'd get from everyone, and to be fair, unless you want to go really fast and tinker with the engine a lot it's best to go with a four-stroke. Kohler, Briggs and Kawasaki all make good, reliable engines, and any would work well. The most common shafts are one inch and one-and-an-eighth inch straight with a one-quarter inch keyway. The one-inch shaft is easier and cheaper to get a clutch for.
The Comet 94C Duster clutch is a good choice in a drive clutch, and they're easily tuned to your application and have a good customer support system.
Hope this helps, good luck.
Don
Stuck in the seventies- not in the swamp.
(6) Attex, a Hustler, a Super Swamp Fox, (2) Tricarts, (3) Tri-sports, a Sno-co trike, 3 Dunecycles, and a Starcraft! ...so far