Well my car is a 1972 Mark III one of the latter Midgets to be produced with a dynamo and control box. I am not sure exactly when they changed over to the alternator, I would guess 1973. Strange they kept the dynamo so long after alternators became available in production, e.g. my MGC GT of 1969 has an alternator as standard equipment.
My Midget is prepared for "reliability trials" and therefore reliability comes before originality (up to a point) provided of course any modification I decide upon is in keeping with the style and specification of the period of the car. So a few years ago when the dynamo or control box gave up, it is difficult to determine which unit is defective unless you carry out tests. I had a spare alternator in the garage and decided to do the conversion.
The alternator is shorter than the dynamo therefore it is necessary to make or buy a bracket for where it bolts onto the cylinder block, I made one out of a piece of 50mm X 50mm X 6mm angle iron (see sketch). The front of the alternator bolts to the water pump in exactly the same way as the dynamo.

Now for the wiring you really do not have to be an auto-electrician. It is much more straightforward than you may realise, it is possible to work it out by putting the two wiring diagrams of the early and late cars side by side. After removal of the control box I think you need to deal with the wires as follows, (but take a look at the two wiring diagrams as confirmation) see sketch :-
1. Wire at E from control box, this is an earth and can be discarded.
2. Wire at D from control box to alternator, no longer required, wire can be removed or insulated at both ends if you do not want to disturb wiring harness.
3. Wire at WL goes to ignition switch through warning lamp, remove connector from control box and join this to the F wire
4. Wire at F join to WL at the control box end, and to the 'IND' terminal on alternator at other end.
5. Two wires at B on control box, disconnect and join together.
6. A new piece of insulated cable is required to join the main positive alternator terminal to the battery side of the solenoid, the grade of the wire required is 80/0.012" (or metric 80/0.3mm) rated 40 amps, that means a cable with 80 strands and each strand is 0.012" diameter.
7. The third terminal on the alternator terminal is for battery sensing also to the battery side of the solenoid. Not all alternators have/need them, apparently the modern alternators are now self sensing.

I usually carry a spare alternator in my Midget on trials events it is much smaller to carry than a dynamo and control box, I have never needed to fit it but I loaned it to friends on two occasions to get them home.

Rob Cull