Thanks to the nudge from 2LZ I finally got around to putting a twist grip throttle on my ATA250D, and I like it. Feels much more natural than the thumb throttle and I haven't had any problem with full lock turns or anything else. The throttle assembly was only $11.53 from
Ebay with free shipping. Came with 2 grips and a cable that was too short so I made a new one. I removed the thumb lever but left the housing in place because it supports the front brake lever and cables. Now all my bikes, quad, and ultralight have twist grip throttles.