The only issue I see with the chain idea is the possibility that it could get hung up in the slotted disc pans and cause havoc. The chain would seem to need to be secured.
If the chain ends were welded, or attached somehow, to the frame in the middle of the space and then under the axle without a whole lot of sag, it should work.
Just thinking aloud.
Scrapers do the job for sure.
As far as welders go, a 220V stick welder is perfect for farm use and use on heavy machinery. I have a couple of Everlast welders and a plasma cutter from them as well. A couple of the old Lincoln "tombstone" welders are in the barn as backups.
With a stick welder there is no worry with MIG gas or the other issues that plague MIG welding. (gun problems, wire feed, type of gas, etc.) But since you have a free MIG welder, it is a good start.