that’s the problem, i can’t think of a legal way to do it.
obviously you can’t get a temp since you can’t get to the licensing office in ontario. i’m not sure about state laws, but here in ontario as soon as a car is signed over to someone else the plates are considered to be no longer registered to the car… so that rules out driving with the previous owner’s plates.
best option if you want to stay in the states is to come down with a truck and rent a dolly/trailer from a uhaul in the states for a one way trip to drop it off in ontario.