If it's not a weak battery, allowing the starter to bog down under load, it sounds like an internal short in the windings. It's not a dead short, but some of the windings are "bypassed", making the starter weaker. Are you hitting it with 12V on the tractor directly from a battery, in order to rule out a solenoid issue?

BUT ... 0.06Ω is not actually out of line for a starter motor, especially since resistance measurements can be iffy near zero ohms. You'd assume that it would draw 2000A+ at 12V with that kind of resistance, but remember that there's also back EMF which cuts down on effective voltage and internal resistance in the battery.