Im assuming your in need of the hst unit all together? And im also assuming its 4wd.
I found this on messicks website. It is not cheap by any means.
There a lot of people on here that are very knowledgeable with these older B series tractors, specifically its hydraulic components. Could you elaborate on what the tractor is doing or not doing and why you believe it needs to be replaced. Often times something as simple as being low on oil or a clogged filter, turned knob, etc. will mimick a faulty pump... at least from what ive read from previous posts.