The gray body looks like it might be 3d-printed; if so they may be making them themselves, or having them made just for them. There have been similar ones here on ES over the years, until the ones SlowCo linked became commonly and cheaply available. (I use two of the ones from the third link for different functions on SB Cruiser).
If it's just the knarp size on the existing cable causing a problem, they have replacements available you can attach instead. One example of many:
https://www.amazon.com/ODYSSEY-Knarps-Cable-Ends-Silver/dp/B0017GM1CG
If it's the amount of cable-pull required to achieve full throttle range, vs the cable-pull length available from the handlebar/etc control device that pulls it, you could change the size of the pulley on the units SlowCo linked (the pulley just sits on a flatted-side shaft and is held on by a single screw into the shaft). There are also other versions that have different pulleys, but they are a LOT more expensive, as they're usually for specific systems (like the Ninebot / Segway stuff); the body itself and internals are exactly the same as the four linked ones, though.