Briggs 42A707 tries to run backwards

I've had this 18hp Briggs for 2 years never been able to get to run. Coil wires where chewed in two from friendly squirrel that had filled every open space it could find with nuts. I have changed the coil went through the carb, (4 screw pump). Never could get it to fire, not even on starting fluid. NEVER BUY A COIL OFF E-BAY! It was defective. Got a new one from MTD. Ok got spark, still nothing.... flywheel pin was sheered. Fixed... burned up the solenoid from cranking it so much, Fixed that. So now it finally runs for a second or 2 then quits and runs backwards blowing exhaust out the carb. Broke the gear of the starter now cause it backfired hard cranking it. I only gave $40 bucks for it, but now I got cash invested in it and would like to use it. Anybody got any ideas??