Chances are the linings are not worn out, just glazed, rusted, etc. Normally you can just disassemble, clean, sand drums and linings, reverse the shoes for a (almost) new surface, (they only wear on one end), replace rusted springs with generic, (hook new and old together and pull to measure correct tension), lube moving parts, good to go for several more years. Tractor brakes get very little use.