9n will not start

I get the impression that it has an alternator on it, like it was at least started toward a 12V conversion, but since you said it's not wired, I assume it's not connected to anything. That's why there's still a 6V battery in it, right?

Anyhow, are your battery cables clean and tight, both at the battery and at the other ends?

Try jumping directly to the starter stud, but like was pointed out, MAKE SURE THE TRACTOR IS IN NEUTRAL. I'd even suggest the ignition switch is turned OFF so it doesn't accidentally fire and start if it does turn. Sounds safer to me, anyway. If it does crank, you can go from there.

If not, check to make sure the starter is well grounded to the block. If the starter and/or bellhousing has been painted, maybe it's just not grounded well enough to pass that much current.

On my 46 2N, some previous owner had put a Ford starter relay in the circuit, controlled by the plunger switch. This seems like a remarkably simple way to prevent that swtich from suffering a painful death.

Anyway, let us know what you find.

Those two conditions are connected. YOU have a bad cable or Connection between the Battery +ve and the Starter Relay. Note: If you had a short in the relay (with a good cable & connection) it, the battery, would vaporize (melt) the short.

The condition you describe indicates that the Ground & Starter are OK but that you have a high resistance on the hot side. The normal voltage drop should be across the Starter. The Cable & starter relay may drop 1.5 volts (read 11 volts on starter stud). When trouble shooting voltage the problem/fault will be upsteam (towards source) from the point where the voltage is lost.

The starter draws >150 amps and both BIG cables and their connections must be good. BTW: The starter will work on 6V or 12V but the ignition will be "very" unhappy if the voltage is not correct.

While you're troubleshooting, Be SURE the transmission is in neutral, and remains there, so you don't run over yourself, if it happens to crank suddenly, and start while you're messing with the starter and associated wiring!.

Assuming the shop knew what they were doing, and the starter is OK, either your engine is stuck, or you have a poor connection somewhere in the 3 large battery cables, since it won't crank, even when you jumper across the neutral-safety starter switch.

If you had not removed the starter, and had it checked, I would say the starter drive could be jammed against the ring gear, but since it acts the same as it did before you removed and replaced the starter, that is PROBABLY not an issue.

Connect the clip lead of a test light to one of the starter bolt heads. Touch the probe end to the BATTERY side of the starter switch. It should light, and remain lit when the starter buttom is pushed. Then, probe the STARTER side of the starter switch... does the test light light when the starter button is pushed? If so, move on to the starter, and probe the terminal stud and push the starter button... is there "juice" right to the starter? If not, it's time to "follow the Volts" as Soundguy likes to say, and figure out WHERE the current is actually getting lost, between the battery, the starter switch, and the starter.

Or, the (+) ground side, from the (+) battery post, to it's terminal, though the cable to chassis ground, though the chassis, to the starter's "frame".