Gokarts4Life-11
The odd one
well you could go with the very popular 6.5hp honda clone engines (can get them for $100 on sale). Now since you want a lower gear ratio so you can go mudding and what not you can probably get away with a centrifugal clutch ($30). In my opinion I'd get a comet 30 series torque converter ($189) it will allow you to use bigger tires and you'll have much better torque and top speed. There's also many mods you can make to a 6.5hp clone to give it more power.