If the axle has a 6-speed gear cluster included then there is no room for additional freewheels.
Replacing the band brake with a disc brake gets additional room as well as only using a single-speed freewheel.
You can get freewheel adapters for a 5/8" axle from sick bikes staton-inc and likely a few other sources.
For disc brake adapters it's kind of a 'build your own' using a 5/8" hub -or- I drilled out the drum of a band brake assembly to accept a disc. The problem then becomes how to mount the caliper without welding stuff.
The least expensive conversion, in terms of labor, is to use a front hub motor.
+1Sancho's Horse wrote:...I am interested to see the differential. The worksman gets its differential abilities from the way the wheels themselves are constructed. They work great, but limit replacement options.