Disco lights. Cool.
It's easy, you can just google SPST, make sure you get one for 12 V DC and that can handle enough amperage, maybe 5-7 amps. Or go to an auto parts store.
One issue I have noticed is that if the revs get too low (like almost about to stall), the voltage might get so low that the switch will turn off (or buzz between on and off) and cut your circuit. Best way to fix this would be to make sure you have a good idle and you should be fine.
If your idle voltage is too low to trip the switch, you could maybe use a 6 V DC relay instead.