That SN would make it a 1973. The HD-6s are still popular and so it is hard to find one for less than 5 or 6 thousand dollars. In good shape, they can cost 10 to 15K. Your proably looking at 5 to 10k to put it back in shape. You should be able to get tractor parts through Fiat-Allis and tracks and rollers through a local track jobber.