For power - go with the twin - plus is you wont need to rev the heck out of it if you get stuck either - i ran a 18HP twin on my mod murray - had tons of power for speed. As much as i had it running around, it didnt seem to use a whole lot of gas.
I ran a 10HP briggs on my agway when it was stock ( came with it) - it did okay for power ( when it ran right), just lacked traction ( turf tires) - im planning on using a 11HP briggs on it in its mod form , should be plenty for what riding ill do with it.
Another mod youll want is a gas pedal/hand throttle - itll give tons better throttle response then the stock throttle setup.
Stock tires will be okay for lite stuff, for more traction go with a AG tire or ATV tire .