The 400 4 rotor fit the crank taper but sits out further, thus if you recut taper you can move it in approx 3/4 inch.
The overall length is shorter, the middle section (that fits over taper& bolt pulls against) is smaller dia. The 750 field coil nearly fits but is long. I think a field coil is fairly easy to make.
Using the stator off a 400, we should find a price. Since dia might? be smaller, and length is shorter, you can end up with a smaller dia custom cover that is tighter to motor..plus, (not sure) it may be able to run dry.