Yep - I know WOODS makes one - Good brand but a little pricey sometimes Product Details
Just google 54" brush hog and you will get some more info. They are mainly light duty (which is what you want with 20hp PTO) units but there are a few out there.
Have you tried running the 48 off center? You're only gaining 6" with a 54 and that is 3" on each side. You should be able to easily offset your 48" by 3" to one side or the other and get it to fully cover the tires on one side. Won't cut down the mowing time (of course 6" extra won't by much at all either) but should be easier to line your rows up.
Get a 60 inch. I ran a 60 inch with a 750 JD. Would work well in most cases.
If the mowing got to heavy and lugging the engine down I just take 2/3 of a swath.
Kubota L3560, Toro 52" and 60" ZTRs, Kubota RTV 900
I run the Woods 54" on my Kubota 2620-a 20hp tractor- and it perfect for the work I do. Most of my cutting are fields that are mowed 2-3 times a year, but I have also been reclaiming some areas on our hunting lease that have not been cut in 8-10 years
Mine does not leave a real nice cut - I run a 6' medium duty and don't really keep up on sharpening my blades as I use it to clear woods and heavy brush frequently. Some say if you keep a sharp edge on the blade it will do a much better job.