I just don't know, Doug.
Is this a "high use" welder, IE did you buy it new?
Sitting outdoors may have caused some problems, but I would not expect so unless it had been water damaged
What I'd try
Save for last, LOL, tear apart the motor and inspect. This is a DC brush motor of some sort, so the brushes / commutator / bearings
Depending on what you might thing the "time" on the unit is, I'd think about a new liner
Inspect clean and maybe just replace the wire roller (s) and make sure you have the right one
Make sure all power connections are tight, at the gun connection, and inspect internally, there's bound to be a couple / three big bolted connections off the transformer / rectifier / filter / etc
You might try furiously working the speed / heat control pots back and forth to clean the wipers. These would be suspect for replacement from water damage.