Okay - Kill switch as wired is a safety, and it is not recommended to remove.
You could get a transistor and connect it to the kill switch and ground and see if that brings it low enough, or you could use a relay. If you want the relay to work as a failsafe then you want the Arduino to energize the relay for the bike to run. That would be using a set of NC or Normally-Closed contacts wired in parallel with the kill switch. If the relay is not energized the contacts are closed and the bike won't run.