well you could go the route redneckcomputergeek went with a power wheels build he did for his son, it was based off a murray chassis but still the same principles apply, turn the transaxle on end, run a shaft from the front to the transaxle input, mount a horizontal shaft engine and run a go cart centrifugal clutch with a chain going to a sprocket on the shaft, there's a little more to it than that but that's the basics of what he did, another method would be to convert to a chain drive straight axle and horizontal shaft engine with a go cart style torque converter, both use engine rpms to engage rather than a clutch