As stated, pull the headstock cover and look inside. You'll probably also have to pull the valve body to really see what's going on, so if you do, note the position of the speed dial and don't rotate the dial or the valve spool in the valve body. Or mark both of them so that you can return them to those positions before reinstallation. Same for the positions of the shift forks and the sliding gears.
The missing speed could be due to a number of reasons. Check the clamps on the shift forks where they slide onto the piston rods in the valve body.
Parts availability is an issue with almost any machine that is this old. If you need a replacement hydraulic pump (there are two), you'll be hunting for a while, and even then you'll likely only get something close and have to make some modifications to get it to work. The Rockford Pullmore clutches can also be a problem in getting parts.
Used or not, the headstock, apron, and quick change gearbox fluids should be changed. There is no actual filter on these Hydrashifts, just two cleanable strainers. One is easy to get to on the back side (either above or below the drive motor depending on the size of your lathe), while the other is inside the headstock and requires major disassembly to remove. The factory recommended just backflushing it unless removal of the headstock gears and shafts for other repairs allowed access to the second strainer.
Check the primary hydraulic pump driven pulley under the sheet metal cover on the left side of the headstock. The keyed pulley ID on the pump shaft is a weak spot, and will start to work under load, especially since the spindle rotation direction is changed by reversing the motor drive (which means the pump drive reverses, too). Once the pulley starts working on the shaft, the Woodruff key gets loose and damages the pulley and/or the shaft.
My Hydrashift had been rode hard and put away wet. The inside of the headstock was filthy, and I ended up replacing a number of the shaft bearings due to wear.