Our 1500Z is about 10 years old. I had the same hydraulic tank leak issue. The biggest danger is that you run the tank dry and damage the pumps, as the oil may leak when mowing and not be seen, and you may not know until it's too late
The oil tank outlet fitting is a very poor design, as any pull the hose has will upset any possible seal. I replaced the rubber grommet but still it leaked badly.
So...I went to a local hydraulics store. The hole is about 3/4" dia, so about the same diameter as the OD of a 1/2" fitting. I was looking for a 1/2" 90° M+F hydraulic swivel adaptor. That way I could insert the male threaded part into the hole and tighten it with some kind of nut from inside the tank. The normal swivel is 1/2" NPT tapered, so not ideal. But they did have that same adaptor in 1/2" BSP parallel (same thread as 1/2" NPT but parallel), and this is better. I also purchased a 1/2" M to 5/8" M hose fitting to connect to the F part of the swivel, and a rubber washer for the inside seal. In the end I wasn't able to find a real 1/2" NPT nut, so used a 1/2" conduit nut. I made a big washer for the outside of the 1/2" swivel fitting where it enters the tank, then slid the rubber then steel washer down and over the parallel thread on the inside, then the 1/2" conduit nut, and tightened all that mess. I had to make a tool for the conduit nut out of a 1/2" PVC conduit coupling-I cut it to look like a castle on the end. To slide all the rubber, steel and conduit nuts down there I used a 1/2" rod inserted into the male end of the swivel. It's hard to work inside the tank. I also put a lot of silicon gasket maker on all parts to help it all seal, and drilled out the 1/2" swivel as it was only about 3/8" ID, and I was concerned that the small hole would cause some pump cavitation and damage the pumps. It seems to be holding up well
I have to say that a fabricated aluminum tank would be better, but I wanted to get it working, and was struggling to find a local fab shop to make this. I also thought it would cost a lot more than $100