How do you change hydrostatic fluid in 1556 fluid? How much fluid does the system hold? Bought this tractor new in 1973.

I have a 1985 Kubota G4200 H, it is a fuel sippin 2 cyl diesel. It came with nothing, learned in New England there isn't much used Kubota attachments for it, BUT I am holding out hope that I can use the Hydrostatic transmission to get a hydraulic lift for a custom 3 point hitch assembly and run...