You're better off building a battery powered one,as you can get it insured, drive it in the park and on the pavement, unless you've got a huge garden or field. I toyed with the petrol idea for a while but went electric, you could mount the engine in the rear you'd need a metal mounting plate to avoid damaging the plywood, i guess it depends on what type of engine you use, bike/lawnmower/generator/chainsaw etc etc which will determine the drive system.