I would like to disable safety interlock on 46" MTD hydrostatic with 18.5 B&S engine.

Before we started could you tell us which safety switch are you thinking of disabiling, there are at least 3 safety switches and some models have more.

Deck engaging handle and the seat.

I don't think this is a good idea but I'm not trying to give you safety advice since you may have your own reasoning. Test both switches to confirm that when you are seated this causes an open circuit. I'd use a continuity tester. No weight on the seat and you get continuity, depress the switch and there is not circuit. If this is your finding just cut the wires to the switch and add two wire nuts. If you find the exact opposite and the circuit is closed when you sit, cut the two wires and wire them together. I believe the deck/seat interface is trying to keep your toes safe when you mount the tractor so be careful.

Yes, the deck engagement switch is one even I don't disable, Hell I will work an extra 2- hours to make sure that one works.

Careful!
Don

PS Had a doctor friend, scary the number of mower related accidents.