If it were me, I think If I had atv suspension to use and a 400cc (I'm assuming you mean 2 stoke) engine, I would make a tube frame up where you sit inside of a cage with the engine behind you. Basically a mini sand rail. Not saying a lawn tractor can't be integrated in somehow, but my feeling is by the time you use a non lawn tractor engine and a lot of other things, it would be so far from a lawn tractor anyway that it wouldn't be worth the work of trying to include it. What kind of transmission or rear axle were you thinking of using?