Just my opinion, but you might want to re-think the twist throttle. When off roading it's typical to be holding on hard to the handgrips. If you get off balance or unseated a twist grip could make matters worse. I ride Motorcycles & trail bikes too and it's fine there, but I think the thumb throttle is well designed on the Quads. Again, Just my opinion.
Just joking. I think there is a company that makes a quad twist throttle conversion called Pro Motion. You may have to look for someplace that sells them for sport quads and then find one that will cross to the Rinny. I doubt you are going to find one listed for a Rincon.
Yeah..Pro Motion... That does sound familiar. I looked into it a couple of years ago for a different quad and they had many different set ups and seemed to be quality and knowledgable. I didn't end up going that way, but that is probably your best bet.