The 2x4 will work fine. If you have a threaded drive shaft that accepts the peg you are way ahead. most of the 31cc weed whackers are ryobi that does have that. The 25cc craftsman i am running has a 5x16 threaded drive shaft. You can still use the bolt on type foot peg you just need a 5x16 fine thread nut and lock washer. You will need to adjust your tension. I would use a solid tie down from bike finder supports if you have them. If you work out a way to tie the engine on both sides of the wheel it will cause less stress on the drive shaft bearing I think. You Don't really need a kill switch I don't think. If you have the right amount of tension on the drive wheel, the motor will just shut down when you get below four miles an hour. If you have to shut down in an emergency just put the setting to full choke that will kill the engine. If you have to while testing.