The tractor would never have run on 70 psi compression. Never.
Redo your test as it can not be accurate.
All plugs out, fresh battery and WOT.
I don't like hunches. Tell us more how you verified the spark.
Old plugs might be fouled etc.
You can burn them off in the kitchen stove or use a propane torch.
You could have so much fuel in it too that even new plugs might not light it off.
I say go back to your spark.
Verify and post back.