Never heard of one. But in looking at the pictures, my first thoughts are a Schlumpf drive on account of the inability to take up chain tension. That would get you a "high" and a "low" with your current set up...assuming you have a standard BSA/English bottom bracket.
If you can swing a derailleur/chain tensioner option, say a claw style, for example, then a mountain bike triple and an E-type derailleur may be an option. You would need to engineer a cable stop though. Ground clearance could be a show stopper.
After that, an internal hub from a tricycle may be an option.
Combining the above can get you many speeds, probably more than you need.
Nothing about your project seems inexpensive.
Edit: It's electric & has a coaster brake. Nevermind.
Last edited by base2; 05-15-19 at 09:15 AM.