Maybe this will be inspiring. Spent 15 or so minutes and modeled up an engine block just going off of photos (so things aren't to scale). I tried to model it in a way that someone with minimal shop access could do it. I'll also explain how i would go about trying to machine this. I should mention, IN THIS WRITE UP, I AM GLOSSING OVER A TON OF HOMEWORK YOU NEED TO DO REGARDING DIMENSIONS!!! Each plate is half inch thick steel. All welds are 3/8" fillet welds. Laser/plas/waterjet (whichever poison you prefer) your 6 plates. This is your chance to cut out the major geometry, don't worry about any threaded holes yet.
(Beer break)
Take your plates and weld them together. Now don't just go blasting welds at it. Now take your time with this. This isn't a race, go slow, take frequent beer breaks, weld short bead lengths, working around the block at different times to keep your heat even.
(Beer break)
(optional but recommended step)-Find a shop with a heat treat oven and have the weldment stress relieved. Or find an appliance store with an old stove. And convert it into your stress relieving oven.
To do that just watch/copy this guy(Beer break)
(Disclaimer...I'M NOT A MACHINIST!)
Next set your big steel block (this weldment will weigh around 100lbs) onto your knee mill with one of the sides (sides=plates with two big round holes) facing up. I would set this beast on some 123 blocks. I would indicate across the top and and shim it until its as close to zero runout as possible. Clamp er down...
(Beer break)
Take a shell mill, fly cutter, small file wielding army of ants, etc. and mill the area around the crankshaft/camshaft holes flat. Then punch your threaded holes for everything.
(Beer break/spray for ants)
Flip the thing over and set the milled surface on the 123 blocks and repeat everything on this side.
(Beer break, and did i mention I'M NOT A MACHINIST!!!)
Set the block upright square everything up by indicating off one of the sides you just milled. Mill the top flat. and then punch your threaded holes.
(bonus round)
Flip the block so its upside down with the milled top facing down on some 123 blocks. Now mill the bottom down until you are taking material off evenly all around it.
After all this go online, and do a long write up about what you did so everyone can tell you how you did it wrong and how you're the worst machnist in the world
Hopefully this helps with this discussion.