I think PBI makes down to a 49 for the XL drums, but have heard of chain clearance issues on the drum as it drops the chain closer to the outside of the drum.
Good luck with that one. Yeah, they make em, but they are junk and eat the drum up, and with little wear, they eat the drum.
As far as going to a disc, they don't stop the bike any faster than a drum, and much more expensive, to get down another tooth or two. Sportsters ran drums on the rear, from the beginning, and front sprockets have always been the cure. If you are trying to get the "perfect" setup, you may have to spend a lot more money and get something that has a disc and sprocket, seperately.
I am willing to bet that the 23 will get him where he needs to be, unless he is drag racing.