Rooster Ignitions offers a solid state replacement. It fits neatly in the original Lucas MCR2 case with the right bolt holes pre-drilled. A little blue loctite and I bolted it back up.
The ignition system was also upgraded to a booster type solid state device. Also provided by Rooster Ignitions, the Rooster Booster fits nicely inside the box. It's strapped tightly to the coil, with multiple Ty Wraps, and doesn't interfere with the ammeter. I even put a plexiglass cover over the aluminum case just in case the Ty Wraps come loose. I don't want it to touch the screws on the back of the ammeter.