What is the number on the deck which is on it now? ie RC54-71B,RCK54-24B
the last three numbers/letter determine the sub model that it fits and the first are the style and width of deck. Switch out the "54" on yours with "48" and do a search to see if there is such a model and if it's available?!!
Did you ever find a 48" mower? Kubota indeed did make one for the B7200.... I know for a fact because I have one.
I'm not currently using it if you are interested in trading your 54" for the 48". I bought my B7200 last year and it came with both the 48" MM and a Woods RM48 3pt mower. I have never mounted or used the MM mower, but the previous owner said it works just fine.