For the life of me, I can't see any good reason why not to use a mag-dynamo, as in the instruction booklet on the original G-5. Other than that Rickman didn't have the parts when they assembled the bike? And the same might have happened when Rickman assembled the other one, that's still in the UK. The 2 might have been assembled at the same time...
Btw, as yours doesn't have lights, I don't think it's an enduro (ISDT/ISDE, like Guesi's) model, but rather a non-road registered MX model. I have no doubt that can also be used in events like desert races in the US, Australia, Africa, or the UK at the time, but I believe proper ISDT/ISDE bikes need to have lights and comply with the legislation for use on public roads. Then as well as now. And while in the UK at the time a 'Daytime-only' use may have been legal, on the continent it most certainly was not, hence
not ISDT.
On the Classic MX races you see Rickman bikes quite regularly, with other engines (just about anything from the period). Best have a look at photos from the ISDT's from the time, maybe I'm wrong.
All MZ's btw.
Anyway, it still leaves you to take a view: stay with the ignition/carb you have, which were possibly fitted by Rickman at the time, or upgrade to something more modern, which is certainly not original, but might work better.