2n starter spins, motor does not.

By the way, I paid 1350, no rust, smooth metal, poor tires, some repair to tranny case just by starter and below battery.

If it won't start this way then it is probably the starter that is bad
When jump starting an N tractor with 12 volts it not necessary to hook the 12 volts to the 6 volts battery at all. As a matter of fact I strongly reccomend against it.
Instead hook the 12 volts directly to the frame of the tractor for the ground and with the transmission in neutral and out of the way of the wheels of the tractor so you don't get run over touch the other cable from the 12 volt source to the post on the starter.
By doing it this way you don't even have to worry about the polarity of the batterys or which cable you have to ground or positive to the frame or the starter as the 6 volt systme is not intered into at all with the 12 volts. Just turn on the switch and touch the hot cable to the starter post and it will go.

I know, whaa whaa. Who knew a 66 year old would need such attention. (ha!)