I looked at solving this darn flywheel problem today. I brought the shredder and the separate 20# flywheel to the bike shop to discuss options with the mechanic. We came up with a few options that might work and I took the whole thing over to the fab shop who did the oven work. I had assumed we would be using the stock Schwinn wheel but the fab guy figured it would be easier to use the flat cast iron 20# flywheel. Unfortunately that flywheel has a very abnormal freewheel setup, so I took it back to the bike shop to figure out how to mount gears on it...
We settled upon using a freehub drive system. This differs from a freewheel in that the latter keeps its ratcheting system in the cogset, and the freehub (aka cassette) has the cogset slide onto a ratcheting "freehub body" on the hub itself. We settled upon welding a steel freehub body (black, at right and in below photo) to the flywheel hub. This has splines that the cassette (left) slides onto. Then the smallest cog (lower left), slides over the end of the freehub body and the lockring (sitting on top of this cog) threads into the freehub body, securing everything together.
This will solve the problem I had last week of the small cog threading itself off. Since the freehub cogs slide onto fixed splines and are held on by the independent lockring, excessive force in either direction is not going to mess with the drive train. I will also be drilling out the rivets that hold the cogs together so I can isolate and correctly space only the two that I will need. The small one will be needed as well, to mate with the lockring.
Because I don't know the history of the flywheel, and critically, because welding the freehub body on will make one bearing inaccessible, I elected to grab new bearings. Thankfully the shop had a pair of appropriate 6001 bearings in stock. I thought it was kind of funny that the old bearings were stamped "West Germany"! But one was a little rusty so I think replacing them was a wise decision.
I'll be working on this more this week and it sounds like the fab shop should be able to take the shredder next week for all this welding.