I guess it would make a lot of sense to adapt to a common pump standard in order to keep this thing viable and "future proof", instead of being stubborn and trying to find an obscure part/solution like usual.
I bet the neighbor down the road could build me an adapter pretty quickly, and I wouldn't see it costing too much. My brother could do it too, but he's generally too busy with his other job.
Another possibility is that the 4 bolts which hold the present SAE B flange onto the clutch are a standard "small engine" pattern. If that's the case, a kit like the one pictured below would bolt right on, with only a small shaft extension needed to bring the Lovejoy couplers together in the middle. I will have to measure the bolt spacing. Using Lovejoy would also solve the issue of mis-matched shafts.
View attachment 224361
At the very least, I'm hoping to hear back from the hydraulic shop today with their pump recommendation so we can discuss.