Try this. Pick up the thumb throttle and have a custome cable made. Any good shop will be able to make a custome cable for short money.
I am running a Honda 400ex throttle opening a 28mm carb. The custome cable was 25 bucks. Any thumb throttle will work just as long as it has enough pull. A thumb throttle opening a 12 mm carb will not open a 16 mm carb fully, but a 16 mm pull thumb throttle will open a 12 mm carb all you have to do is adjust the throttle stop.
Trikes:
1985 ATC 70 mod to 184cc
1984 ATC70 mod to 124cc
1983 ATC 70 Xmas restoration
Quads:
1986 TRX 250R
2002 TRX 400EX
2- LT80 for the kids
1986 LT 50