Relay conversion simply dispenses with all the electronics and safety. The relay selected needs a decent current capability, I'd suggest 70 - 100 amps because of stall current pulled., using the small switch to operate the relay and pass the current.
Tbh if your not to sure about the basics then seek help or don't do it, or you could easily have a fire or other nasty accident.
Your one of 'us' now sonprecious, and plenty of help on here to be had
.
Alan
C5 bits come up all the time on eBay, so a nit if patience all will be well.
Alan