Welcome to the forums. Yeah, you can rig up another key switch from another ATV or even a lawnmower or tractor. You would just have to figure out the wires and it would depend on the key switch, but it can be done, because I've seen lawnmower and tractor key switches rigged up on them before.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.