Absolutely, there are auxiliary power plants available as kit systems. They're pricy though, and I've seen very few that include a driveshaft, so you'll have that cost too. The pumps alone can be forward of $1000, and you still don't have anything to attach it or a reservoir of oil, or hoses to connect in between. I would still recommend buying an assembled system unless you have plenty time on your hands and have a lot of different skill sets in hydraulics, welding, plumbing, etc. etc. Ask yourself "Is there any part of this I cannot do?" If the answer is yes, make sure you know someone who does, or plan for failures. By the time you finish spending money to avoid buying a splitter, you'll probably have spent enough to buy 3 of them. And one other thing to consider. When you get a good one, you're gonna want a better one, especially if you start splitting for friends/relatives/money. You can spend as much money as you want on most anything tractor related.
Like many others I went down this road, too, trying to reduce the number of small engines I have to maintain. While everyone else is telling you good information about pressure and flow to the cylinder, I haven't seen anyone bring up the amount of horsepower required to do that. A large cylinder that needs a lot of pressure and flow is gonna take a fairly large prime mover and pump to do it, assuming you want something that will split anything you throw at it. Hose size will affect flow, as will valve selection (port sizes on the valve) and manifold configuration. If you see one that will do 20000 pounds of force with a 6.5HP engine on it, it probably won't be very fast. Pressure is just that, pressure, and the relief on the pump will prevent it from stalling the engine, meaning the ram just stopped. If you get high pressure from a small engine, you typically won't get high flow.
Building a splitter from scratch can get very involved, as well as dangerous. When things break with 40,000 pounds of force on them, there's usually significant collateral damage. If you have an old one that needs a new powerplant, then a PTO pump and auxiliary tank may not be a bad idea. Unfortunately, you'll have to something about moving the tank and the splitter. There are 3 point splitters out there, but I strongly advise against one that shares oil with your tractor. That's a big cylinder, and a wonderful place to collect moisture and other things you don't want in your tractor. That's a lot of oil. An external pump is the way I'd handle it. The cost of a 3 point hitch mounted splitter isn't too far off the cost of a good premium grade stand-alone. I've shopped a lot, and I don't have one simply because I don't have a fire place. I just cut the rounds short enough I can split them fairly easily with an axe, or hammer and wedges if absolutely necessary.
