I made an executive decision today. Too many irons in the fire, so I decided to prioritize and simplify.
Focus on building the 550/650 hybrid, since the engine is close to completion, it will not require one-off, machined wheels, and I have all of the build components (minus carburetors and shocks) at the ready. Sell the RC51 this coming Spring (don't ride it enough). Sell the black 750 (have a couple of buyers in mind) if it makes sense. Keep the stock 550 for putting around and vintage events, and ride the bejeezus out of the hybrid.