Alright, I scaled the engine, it was way too big in the first concept above.
Below is a shot after doing the engineering on the stand. I just did it with the old fashioned stress and deflection equations (statics and mechanics of materials) since I trust the tried equations more than my ability to set up solidworks simulations with assemblies at this point. I've got parts down but assemblies take more. Also, I don't have fancy geometry, perfect to keep my math skills honed. I put a screenshot of some of the 6 page mathcad worksheet I used to to the computations. I'm lucky to have access to mathcad since it makes iteration a snap, but all this could be done with a simple calculator and paper.
I'm glad I did the math on the main shaft that supports the engine. The 1.875 heavy DOM I wanted to use would have had 80,000 psi in it! Stepping up to a 2.5" Od with pretty much any ID gets me below 36,000 so I don't need to worry.
Better to take a few hours doing the math than be rewelding and adding material in the shop. Or worry about it coming down!