Get on eBay and search for pit bike start button and you'll see lots of options. The simplest ones have just 2 wires like this one,
http://www.ebay.com/itm/Universal-St...VWxmzZ&vxp=mtr
Different bikes use different wiring methods for the switch and solenoid, either breaking/completing the circuit in the battery + side or the ground side, the ground side being the most common.