G1900's have a drive shaft that runs from the engine to the transmission. The hydraulic pump is on the transmission so if the tranny ain't turnin, the pump ain't workin.
So, what you can do is climb around under there (engine OFF!!) and grab the drive shaft. There are two, one runs the deck the other one is buried up higher, between the frame rails, way up in there. Grab the shaft and see if you can manually turn it independently of the engine, e.g.--if you can spin the shaft and the engine don't turn, you have a problem! At that point you can then figure out where that problem is. There are two u-joints, and they are known to fail sometimes. There's a yoke on the transmission and another one on the engine, they both fail sometimes (strip the splines). If its' the trans end yoke, pray to the Kubota Gods that it didn't hurt the trans splines. If it's the engine end, replace the yoke, AND the 3 bolt flanged stub shaft on the engine. While you have it apart, go ahead and replace the front and rear u-joints too if the shaft yokes ain't tore up.
In order to get the shaft out you have 3 choices, easiest to hardest in this order: (1)-remove the engine from the frame. (2) remove the transmission from the frame. (3) remove all of the steering, the steering box, fenders, floor, radiator, etc and yank it out the top.
while it's apart throw it on a trailer and take it to a car wash to pressure wash it unless you have one of your own. It'll be messy.....especially around the transmission area from all the years of spilling diesel.
Years back on these old antiques it was often that I'd buy them cheap because the owner thought the motor was rattling, so they'd sell them as "needs engine work". "Usually" the drive shaft was trashed. Replace it or the necessary parts (flange stub shaft, yoke, u-joints) and it was back to brand new feeling again. When the u-joints and/or yokes wear out, they usually develop vibration. This happens over a long period of time so often owners never even notice it until it just finally swarms.