If you can retract the front idler, (it might take a come along) you might be able to drive it back on. Chances on a machine that old, everything is well worn, (also part of the reason it came off). If the track is on the outside of the front roller, slowly drive & turn INTO the track. The opposite if the track is on the inside. It helps if you have some helpers and another machine with a chain (on the track) to assist things. At this point, you really can't make things much worse (just don't get it off the rear sprocket as well). You'll also need to decide of forward or reverse is correct. It's not rocket science down there, just look at it and decide how you need to move the machine to put the track back where it belongs. It might be worth a call to a local contractor or mechanic. They might have it back on in a few minutes if they have done this before.