Friction drive kits and Gopeds use thumb throttles.
my 1st motobike came with a bad throttle (it was the typical, over-the-bar twisty motorcycle type) so after much anguish, i decided to use a brake lever with a joining coupler for the cables. As long as you get the tension correct and it has a good spring, I'd highly recommend using one! I also had a single brake handle that has duel ports (so it pulls both front and rear brakes) aligned just over the throttle, so you can't brake while under load. worked flawlessly, then it got stolen.
I used a brake lever as throttle control on mine! It works perfectly, just get used to the way that it works.