Doug came up with a minimalist idea that saved reinventing the wheel, as it were. The bottom bracket tube of the Motobecane has been extracted from the rest of it, and cleaned up, with the shoulders of it skimmed a little to allow ball bearing cups to have a close, tight, fit. The plain bushes the pedal crank runs in were extracted, the tube was shortened, and the bushes reinserted. The complete assembly is then placed between two bearing cups back in the original bottom bracket, using all original Motobecane parts for the mechanics, but looking 100% vintage bicycle externally.
Here is a sort of sketch.
