Take the housing off and blow out the twenty years worth of dust accumulation. Inspect the wire for corrosion on the outer wrap, and consider getting a new spool if there’s visible corrosion.
Make sure there’s gas in the bottle, the gas delivery hose isn’t cracked and leaking, and if everything looks normal, give it a shot. I wouldn’t think a transformer welder has much that can go bad from sitting for an extended period….it’s not like a banana, with a limited shelf life.