Levsmith...
A push mower is typically a vertical shaft engine, and they are a dime a dozen as they say.
The only problem with using a vertical shaft engine is how to transfer the power vertically to the horizontal axle or sprocket. From my experience, it is not so easily done!
My pusher is a 6.5hp horizontal engine, BUT I would say you could easily get very good results from a 3 or 3.5 horsepower engine....
Any thoughts on this znsanerider?