I don't know if this is true, but someone told me that the 21 and the 19 are the same, but that one has been governed, so that it was not as responsive. You may only need to make some carburator adjustments in order to get more power.
id go with the 23 ..but the 19 will do good on the 48...the more power the better but if you get a extra good deal the 19 kaw has plenty of torq for a 48 inch deck...i had a 19 on a lazer hp 48 inch ...it was plenty of power.
I bought a Scag Tiger Cub last year with the 19hp and 48" deck and a factory bagger. It has no problem bagging or cutting. With all the rain we had last year, I slowed it down when it was wet. The price was to good for this combo. To step up to a 23hp and 52" deck would have cost another $1400. Hope this helps. Matt