John,
It really sounds like a head gasket issue to me. In fact, hopefully it is the head gasket and not a crack somewhere. You could have either one and not get coolant in the oil, or vice-versa.
If you want to try to get really clever before pulling your cylinder head, you could take an old radiator cap and braze a tube to it. Then run a hose from that tube into a bucket of water. Run the tractor. If you get continuous bubbles, the you are getting combustion gas in your coolant.
Sounds like you are going to have to pull your cylinder head at a minimum. With luck, you can just replace the head gasket, and not have to have the top deck of the cylinder block and/or the head deck planed.
Good luck. Keep us posted.
JB