I don't want to have to worry about if there's enough battery to get me home anymore, and I want to run the lights all the time. I also don't want an after-market alternator conversion that does not look like the original.
The plan is to create an alternator conversion that produces 6 volts, is fully encased in the original housing, and works reliably.
I've seen others who have used a Kubota tractor permanent magnet alternator to do the job. Its only 3-1/2" in diameter and shorter than the MC45L dynamo. That means it should fit inside the casing. The trick is to mount it up.
It comes as a flange mount unit for belt drive, with its own bearings.
 I actually did use the bearings after all to keep the stator assembly centered in the magnet. The magnetic forces are VERY strong radially, and will pull the stator into the magnet if there is the slightest mis-alignment or looseness. A spacer piece was machined to hold two of the bearings into the stator and then the bearings were placed on the center bolt with their own machined spacers.
The permanent magnet rotor can attach directly to the S7 engine crankshaft, and the stator can mount to the engine block, or dynamo casing. All that is critical is that it runs centered and true. Should not be a problem for a guy with a lathe in his workshop.
Here's the sample fit up. The center bolt presses out of the rotor cup easily. This will allow me to make a bolt (left-handed Whitworth of course) to fit in there and mount to the crankshaft. The question is whether 'tis better to bolt the stator to the engine block, or to bolt the stator to the outside of the dynamo, and put the rotor cup "upside down" against the crank. I'll have to make an adapter to provide the seal to the front of the engine regardless. The stator to the front makes it easier for the wires to come out to the terminal block on the dynamo casing.
Here's a VERY ROUGH sketch of the assembly. I keep changing the design, so my dimensioned sketch is pretty bad. I will require a minor modification to the dynamo case. The cap screws will interfere with the side of case, so the case will need to be ground away in two small spots not visible from the outside. The cap screws can be modified slightly to reduce the amount of grinding to the case.