Hi Mike and
to the forum.
Most of the GM controllers only have a basic On or Off switched operation for the regen.
To achieve variable regen, you would need to have a controller that is designed to support it, along with some form of brake lever
(or thumb throttle) with a variable voltage output signal to control it.
As you will no longer require a clutch lever on your converted bike, perhaps this lever could be adapted to control the level of regen in conjunction with a controller that supports variable regen.
Alan