A tractor that "goes on its own" sounds like an accident waiting to happen. If the engine has a magneto type ignition, there will be an electric pulse generated when the flywheel is turned-that is basic physics. If there is no ground wire to discharge this pulse, then there will be a spark generated, and the engine will run, if it has fuel and air. It sounds like the magneto/ignition ground circuit is defective.

***" It sounds like the magneto/ignition ground circuit is defective."***
I'm not inferring that from what was stated by the OP.
I believe the OP means that as soon as the engine begins running, the tractor begins moving across the ground.
Are we on the same page here?

Sounds like the belt may be too short?

Bill - methinks the OP recently swapped out engines on this rig which could toss a whole bunch more iffy's into the mix... just a heads up.

"Sounds like the belt may be too short?"

Or miss routed.

Of course the OP told us nothing about the name. model etc of the tractor, so since there have been hundreds of models made in the last 50 years we are shooting into the wind.

For some folks, finding and utilizing the resources of this forum is kind of sort of almost like a dog chasing a car.............what the heck would he do with it if he caught it?
Don't ask me why I think like this, I am too tired to talk about it. :^)