Here is a link to a dealer in KY. that will sell an Allied WC3, already mounted on a 550G, but will sell separate. No price though.
CASE 550-G for sale in Middlesboro, Kentucky - EquipmentTraderOnline.com
I Googled "Allied W3C", and found several. One was $3500.00. You'll probably have to buy the controls though for your dozer, other than the one listed above.
We had a 550G with an Allied winch. While it was OK, I much preferred the Carco winches we had on the other dozers. Single lever control, and you had power out, power in, and free spool, with a little tension. Being we did a lot of creek channel work, it was great to tie yourself off to a tree, and free spool out, but yet enough tension to keep the cable from getting into a birds nest.
With the Allied, when it free spooled, you could just about bet you were going to have a mess on the spool. It would totally release, and most of the time wrap around, crossing the cable over itself. If you weren't in a position to unroll all the cable, then respool, the cable would pull itself under itself, and you had a mess, plus crimping/kinking the cable. A guaranteed breaking spot in the near future.
I Googled a "550G with Carco Winch", and a model Carco 30B seems to be the one for a 550G.
Winches are rated by lbs. of pull. They will fit about any dozer, you just need to buy the mounting and control accsessories so as to fit a particular dozer.
Just a quick look, and found Carco 30B's priced at $7500.00 used, to $27,500.00 for a new one.
At least you have a couple of model numbers to go by to enter into a search, and do some pricing.
Carco 30B
Allied WC3
Hope this helps..!!