I built this one for my dad 20 years ago, he on the 4th motor. plumb wore out the other 3 motors , the 1st one was totally rebuild, the other 2 new motors, this present motor was new also but is OHV motor so he modified the splitter so it could run this newer motor. I had rebuild it, clean it up, new paint in the attached pictures below. My dad is just plain hard on equipment. I build this for him when he retired if you call it retired, he was cutting 5 cords a day back then. He's an old logger, its just tough not having a saw in his hands.
So, this is mostly build from scrap steel I purchased at the metal recyclers. The cylinder is off a backhoe, I build the tank from some square tubing, I welded in some baffles for the return oil to spray against to help cool it. You need a tank large enough the oil to cool. Northern Tool sale a lot of splitter parts. I bought the pump, the valve, oil filter from them. I build my own pump mount but Northern sales a mount. I bought a 2 stage pump that step down to 5 gallon min/16 gallon min. This splitter pump never steps down to the 5gal stage. Therefore I wish I'd bought the 22gal pump. The 16 gal can move the cylinder six 36" strokes a min. I think it cal. out to about 46,000 to 48,000 psi. I know my dad so I welded the controls to where he couldn't reach the splitter and run the control, but he had a friend move it right after he got it. It needs a 4 way/6 way splitter adapter and a lift arm if I was to redesign it. Dad never use it in the vertical position no matter how big the rounds are. I made the beam so the wood can drop and pile up below the splitter and the slitter can be moved forward as it piles up. Dad cold deck his logs. Works in front of the deck.
I never run this much so I sure there is some other design changes that would work better. Oh and I build the muffler, I bought 3 commercial mufflers cut them and weld them back together to save weight and quiet it down to reasonable volume. Make sure you add in the charging system to keep the battery level good, the 1st motor i neglected to do that and that was always a pain for my dad.
If you can weld and have some fab skills they aren't to bad to build, but Northern Tool sale all the parts you need, tank, ram, pumps, filters, couplers (oh don't use a love-joy coupler for the motor/pump get 2 sprockets and a double roll chain). The hoses I just took the splitter to shop and had them custom made to make sure I bought the right fittings and hose lengths. I also recommend transmission fluid over anything else.