I cant remember exactly what seal I used but it was just a standard metric one from the local bearing place. 20mm ID and probably 30mm or a bit more OD. Cost a couple of dollars.
Its a simple mod if you have a lathe. I turned up a dummy shaft that was a nice fit in the bush, bolt the case to a face plate and clock off the dummy shaft, then simply bore the case to the OD, minus about .004" for interference, and width of the seal. You dont need to remove the bush.