Ok, so you replaced the filter?
Does your drive still work? do you have heaps of power etc? is it just the hydraulics giving you trouble?
You can get a re-conditioned double transmission pump for about $2,300 and the vane pump cartrage is around the 200 mark, not sure if it comes with the transmission pump or not.
Look at:
http://www.skidloaderpumps.com/servlet/the-Bobcat-cln-before-1990/Categories
On the second page it shows the hydrostatic pump. Its a TA1919 pump with the hydraulic pump stuck on the front.
Like i said, i would start by checking that the hydrostats work, if they do you can look at the hydraulic pump. Its easy to pull apart, look for wear in the form of bumps in the cam plate (the centre section the vanes run in) Small scratches will not be a problem as the vanes wear into these and the pump works just fine. But if you have bumps in it, the oil will by-pass causing lower pressure.
What sort of crud came out of the filter? If its the hydro pump or drive motors it will be metal, if its the hydrostats it will be brass.
Don't go out and buy a new pump because you *think* its broken. These pumps can be brought back to life with a few hours worth of elbow grease. The hydraulic pump can be repaired for around $200.