Is it 12 volts or 6? On my 1951-6, I had a very similar issue this year. Luckily, it turned out to be nothing more than dirty terminals. I took them off, wire brushed, applied a little di-electric grease, re-tightened, and it has started great ever since. My tractor has almost 2000 hours on it now and the factory/starter, alternator, and wiring harness are all original.