All is not lost. Pull the front cover off the engine (by the alternator) and see where your alternator brushes presently are running on the rotor slip ring. If the crank has slipped ahead the carbon brushes will be contacting to the rear of where they usually locate. If they are still running in the same spot as usual then the crank hasn't moved and the thrust bearings have not come off their perches. If that's the case, then now is the time to block the crank at the front of said alternator.
If your level of experience is limited with these machines then maybe you should take a pass on doing the rear seal and stick to the oil pump cover.