Who put the engine in the tractor? you? Once the fuel was disconnected at the 1st run, transported & hooked up again, the air needs to be bled out AGAIN.. or it will never start..
Does it have ant "safety" features on the machine? They are disconnected or powered up on a dyno run.. {inoperative}
BUT once back in the machine, they are hooked back up.. neutral safety switch maybe??
I'm leaning towards AIR in the system.. especially if it ran before install..