One thing you can be sure of - it is from the cooling system!
There are only a few places, in any engine, at which coolant can leak into the oil.
It could be a head gasket; a cracked or porous casting - head or block; a low pressure oil cooler through the rad; failed liner O ring seals; core plug leaking; push rod tubes; cracked or porous liner. Might be one or two others I have forgotten and not all are appropriate to this tractor.
The most likely failure on this tractor is the O ring liner seals.
Seems like pan off, and pressurise the cooling system if necessary, to spot the source of the leak. Hope it is a core plug in the head, if they are present. If they are, only removal of the rocker cover, not the sump, would be the easier starting point.
Engine bearings may well need to be checked out anyway, as antifreeze in the oil soon destroys them.