for Honda GX270 or GX390
Nope.
There's riding mower stuff that doesn't like to spin fast and tends to break if you shift it like a car...
And people have taken blown bike engines (small stuff like a 200cc 1cyl) removed the jug and Piston, and machined a custom input shaft to go in place of the crankshaft...
Or there is the divorced Harley style transmission, which will set you back more than most of our go karts cost in their entirety.
But there's really nothing like a 4 speed with reverse standalone trans that will bolt up to an industrial engine.