I didn't know the particulars of the Justy's layout. It sounds like an opportunity for a 'through the road' hybrid. With two more or less independent drivetrains, there's less integration needed. If you have a running vehicle that you needn't rely on, you might be able to keep it running through the hybridization project.
Forklift DC motors were all that was available just a few years ago. The situation now is that the junkyards are filling with OEM EVs that are a source for motors, inverters, chargers and battery packs that are miles ahead of the previous sources, and 1/10th the cost. For my own EV project I got the
Toyota MGR.
Here's a picture comparing it to to a 36hp VW engine case:
A 98lb, 68hp, single-speed 10,000rpm device. It works with an open source controller
you can build yourself.