Found the Worksman Stretch Mover™ M2620-3CB-STR Industrial Tricycle Owner’s Manual:
https://www.worksmancycles.com/media/MANUALStretchMover.pdfand the Factory Direct Parts List, with prices:
https://www.worksmancycles.com/media/2018/2018factorydirectpartslistwithmaintenancepresentation.pdfYou seem to need part# 404SBST: a brand-new 100% reliable drop-in hub with *double sprocket*. Perhaps you don't have $175 (more with shipping) burning a hole in your pocket, but that's WAY cheaper than the sort of custom design and fabrication you're envisioning. You won't find another double sprocket hub like this that you don't make yourself, and you can't make one yourself for less than half a grand.
This vid seems to refer to a slightly newer model of your hub, it's probably pretty similar inside:
https://www.youtube.com/watch?v=fpW_QzSc-HAThat's a daunting process, especially without a bench and a vise, but easier than attempting to manufacture a double sprocket hub and it's support structure out of a Nexus 8. The only direct cost would be a handful of grease and some solvent.
