I would guess the head gasket is leaking. I ran a W9 IHC tractor for years with the rad full of diesel fuel, the cracked head was too expensive to fix. As long as you keep a close watch on the temp guage it won't hurt anything.They do run hotter with fuel oil in the rad, not as efficient I guess. Might be a fire hazard if it gets to boiling!
Don't be too quick to condemn any old tractor as "not worth much". Go to a few auctions and see what they go for!