I have the 20hp Kohler Magnum in mine. With a hard tire like the Terra 26" I haul tail on the trial with my butt well off the seat, knees bent.:help: You will smoke any 16hp on any decent dry terrain. 30mph on flat dry stuff and grass.
At a crawl (how I drive in the rough and off trail), I don't think there is much difference between the different hp of motors. I have seen a little wee briggs on another rider's old orange 8x8 argo with little bitty muck-donuts do anything on the trail, including mud wading.
If I bought a new engine, I would get an 18hp or 20hp Honda, if I was made out of money I would get one with efi. Kohler parts are really spendy, that's their main drawback in my opinion.:dontknow: