I'm pretty sure my mzt-52 has a sticker on the ROPS saying it also fits Dixon ultra series.
Thanks for the reminder!!! You are spot on that Dixon & Husqvarna are interchangeable. The Dixon Ultra is basically the same chassis as the Dixon DX-100 series(their entry-level Commercial). That being said, after Husqvarnara purchased Dixon a few years back they(Husq) implemented a significant amount of Dixon commercial-type ZTR's into specifically the M-ZT Husqvarna ZTR series. Spec-for-spec, the Husqvarna M-ZT 52 is more similar to the Dixon DX-100 than it is to the Dixon Ram series and I consider it one of the "best buys" in my opinion of ZTR's to purchase.
BTW, in 2012 I almost bought a Dixon DX-152 which I think is almost identical to the Husqvarna M-ZT 52, but then I looked at the Gravely's and the dealer gave me such a great deal I've never looked back.
P.S.
So just find a local Dixon or Husqvarna dealer and the OP should be able to get a ROPS for that Dixon Ram Ultra.