Building my own 3PH mounted splitter was an idea I was kicking around too, but what everyone else is saying about the old fluid in the cylinder is exactly what made me shy away from it, and even squelched the idea completely. The next caveat with the oil is that not all seals are compatible with all hydraulic fluids. That would be a bad thing to find out as you're splitting a piece of wood and oil is spraying everywhere. It's already a 'sealed loop' that doesn't need your tractor's oil. Keeping it that way is the safest thing for your tractor.
The parts to connect it to your tractor safely are very likely going to cost more than either repairing the engine that's on that splitter, or putting a new Predator on it as suggested. That's a Briggs Industrial engine, so maybe it only needs a new carb. The fuel shut-off valve just below the tank looks like it may be shut off. If that's the problem, you probably got a dynamite deal on that. Looks like they at least kept a tarp over it to keep rain out of the gas tank and hydraulic tank. HF has new tires already mounted for about $30 each. Or, they also have tubes you can put in those if you just like the torture of handling tires that small. That really looks like it's barely used and doesn't need a lot to fix. If there isn't a hole in the side of the block of that engine, and it will turn over at all, I doubt it would take more than an hour or so to get it running. It would probably take longer to change the oil in it and purge the hydraulics with fresh fluid. If the pump is locked up, you will probably still be miles ahead replacing the pump. One has to wonder if that's the actual "engine" problem. Take the pump off the engine and see if it'll turn over.
I wanted to find one similar that I could buy an external pump to use with it and adapt it to my 3PH. That type of splitter is very expensive because of the dedicated pump. But, that might be another option if you can figure out a way to adapt the existing pump and get it connected to the PTO, however, you're going to want to make sure you verify and test the pressure relief. It would definitely be over powered by your BX, with some risk of hose rupture or pump grenading if the pressure relief doesn't work. Then, my next idea was to get a rated pump to use at the PTO, but they seem to cost nearly as much as that splitter in the photo. SO, plan B shot down.
I know WHY you want to do the conversion, and WHY I decided against any splitter that shares my transmission/hydraulic oil with my tractor. I could buy 21 of those splitters for the cost to replace my tractor. Considering one 5 gallon bucket of SUDT2 is currently nearly $100 here, I can't imagine it being much better where you are. Same $100 will get you a brand spanking new Predator engine. $30 will get you a new carb, spark plug, and air filter for that Briggs. It may well be worth the effort to try repairing what you have first.
The Predator I bought for an old Troybilt tiller cranks better and runs better than any Briggs engine I have. I turn on the fuel shutoff, set the choke, set the throttle, turn on the kill switch, and normally, first pull, it cranks and runs. That's hard to argue with. When I know I'm not going to use it again for a while, I just turn off the fuel and let it empty the carb bowl before I roll it back into the shop. 3 years later, it still runs like a scalded dog. It also increased the HP of the tiller from 6 HP to 6.5 HP. Not a huge increase, but with OHV, it runs quieter and better than the old original Tecumseh ever did. The rings on the original engine are stuck and the blow-by is terrible. I seriously doubt I could do enough to it to keep it running because Tecumseh no longer supplies repair parts for that model engine. I hate to do it, but looks like the old engine is on it's way to the scrap yard. The tiller was my mom's, and has some sentimental value, which was my sole purpose for restoring it.
Just chiming in, but it's your splitter and your tractor. Do what you think is best for you.