You can purchase sheet cork in various thicknesses and this can be used to make a seal/spacer for the Alton. To do this, cut a ring of cork to fit between the Alton and the flange on the engine casting. This cork seal will be a narrow ring laying right at the outside dia. of the Alton. If you look at the Alton and look at the engine casting the dimension for the dia and width of the cork will be obvious.
You may have to do some trial and error with different cork thicknesses in order to get the appropriate thickness such that when the Alton with cork fitted is pushed up tight to the engine casting, making a seal, the sprocket is centered on the width of the primary chain rollers.
It has been suggested to me that it would be a good plan to put a tiny groove across the face at the top side of the cork seal in order to allow primary case air pressure to escape as things heat up. Otherwise it is possible that positive pressure could be created in the primary, and this could cause the primary case to leak oil.
I did not put the groove in and I have had no problem with primary leakage, however.
In fact, I used the cork seal described above on my Miller to cure leakage I had around that unit. When I switched to the Alton, there wasn't room for the cork since in the case of the Alton, the sprocket sat nicely in center with unit pushed up tight, metal to metal. I used a heavy bodied anerobic sealer on the flange instead.
It sounds like your situation with the Alton is similar to the way my Miller lined up, so some thickness of cork should fit the bill.