Mower barely runs and has blue exhaust
Blue smoke means it's burning oil.

This could be from you overfilling the crankcase but if it is an older engine it may have been burning a little oil before the oil was added.

The key here is, was it running okay after the oil change? If the problem started afterwards it then that is the likely cause.

Most small engines like that don't have an oil pump and would have no way to get oil past the head gasket. Even automobile engines don't normally suck oil into the cylinders past the head gasket. It is more likely the piston rings and valve seals are worn so badly that they are allowing oil to pass into the cylinder. Especially if it is overfilled. Watch the first part of this video about splash lubrication. Most older lawn mowers would use this method.

Your first step should be to verify that the oil is NOT overfilled. It could be forcing it past the valves into the cylinder. Your tractor data says it takes 1.8 quarts. If you put more than that in it, then you overfilled it. Your problem could be fixed with a proper level of oil in the crankcase.

Good luck!