1986 Toyota Tercel Wagon, 4WD

My son took the fuel tank drain plug out to drain the fuel out of it and didn't put it back in. I found this out after towing it here to try to fix the clutch. The plug is pretty permanently lost.

The problem is I don't have a clue what the size or threads are for it. I've spent some time searching online for a tech manual, but I can't find one.

If anyone has any technical information on this car I'd really appreciate knowing what size plug to buy to replace it.

Thank You.