Hi Wind,
The standard sprockets for the A65 are 18T for sidecar work and 20T and 21T for solo. The slots are designed so all these sizes will go through. However, SRM produce a 22T and used to do a 23T. Mine is a 22T which is great for keeping the revs lower at high speeds but previous owner has cut and filed extra slots, one being at the bottom so close to the plate mounting hole that said hole has disappeared leaving a great leak path for the chaincase oil. BODGE!!
My advice is, don't mess with these slots. If you have a 22T+ sprocket and need to remove it, pull the gear cluster out from the other side, a relatively simple task as the cluster comes out in one assembly whilst still attached to the round gearbox cover.
Chris