Welcome to the SBO forum.
I would not leave the engine on the transom, especially on an 800-mile road trip. I would drain the fuel tank and set the engine on a carpet scrap in the cabin. I would strap it down so that it stays in one place. I would put a lock on the cabin door.
The engineers did not design the transom to hold that type of load bouncing about over varying degrees of rough highway. Not to say anything about the temptation of an engine hanging about on the back of a trailer in a parking lot is for would-be thieves.
It is your boat. Let us know how you made out on the trip.