Find and buy some Kroil. If you can't find it, use PB blaster. WD-40 is not really a penetrating oil, even though it suggests it is. Unfortunately, freeing up stuck linkage is time consuming, and may even require disassembly to get it freed up. Takes huge amounts of time and is not fun. Good luck.
I'd either remove the engine from the frame and work on it, or worry about getting it running after the linkage is free. Good motor does you no good in a tractor you can't drive anyway, so fix the known issues first. It would be different if it was stuck in neutral. Most likely it would be best to get at the linkage from underneath. I'd drain all the oils and hoist the front end up in the air to get at it easier.
The gearbox on the back is only for a tiller. The hitch used to raise it is compatible with all sorts of implements (in conjunction with an adapter).
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