I'd say that a head gasket is fairly manufacturer specific, and finding a after-market supplier is doubtful. However, the 1500 engine probably is made by Shinbura (Never can remember how to spell it). Some dealers have contact with the manufacturer in Japan and can get engine parts that for Shinbura tractors that are different from the Ford parts. However, I imagine that would take forever and wouldn't cost less.
Parts for tractors may seem ridiculously expensive. However, it's good to keep in mind that we are paying a manufacturer to keep a supply of parts available for machines that may be out of production for more than 30 years, and there weren't that many made anyway. It may be a big premium to pay, but I want to keep my mid-80's tractor going.