Be aware, there is a hose that connects to the other side upright, as it also part of the same hydraulic reservoir. Check for a leaking hose, and/or fitting.
Hopefully, it's just a leaking line, or maybe the valve underneath leaking. I looked the tank up at Case Construction:, and they show it N/A. Googled the part number, and found one used one for $400. If it is in fact the tank itself leaking, I'd have to guess a good radiator shop could patch it.
I know on mine, the throttle shaft seal was leaking on mine. The top of the injection pump was wet with fuel. Luckily, a local diesel supply had the seal kit in stock, 3-4 years ago, for under $25. It "probably" would have been an hour or less job, but as my luck sometimes runs, one of the screws holding the top cover on snapped off, due to rust. It snapped off just under the head, and left enough that I could get on it with a small pair of vise-grips, and turn it out. I got replacement screws at Fastenal, with a internal hex head, so as to use a small allen wrench to install/remove after that, instead of the Fillister type heads. Put a good coat of nickle never-seize, when I installed the new one, mainly on the stem that goes through the cover where it snapped, and a smidge on the threads.
If you plan on keeping it, I'd advise to get an OEM service manual for it. I got one off ebay for mine, slightly used, but in good condition for $75. Well worth the money, when doing repairs. They are about 4" thick, and very detailed, and lots of actual pictures of a procedure, makes life a whole lot easier...