Leeson, Kawasaki, and others sell a 50 amp add on kit that boosts the gasoline engined Mules to a total of 65 amps.
I have used these, but they are only for low speed operation of high loads. Put one on and go riding along with a ton of lights and you will snap the end of the crankshaft off as the engine is not really built to handle drives from the wrong end like these require.
I recommend using an additional battery for occasional high loads, then just put it on the maintenance charger when parked, or buy a diesel Mule and get a decent charging system right off the factory floor.