The mag can be converted OK, but there is a little bit to do.
Change of end-housing at the cb end as mentioned;
change of camring for one with notches in it to allow for movement and to fit the operating plunger (you could mod the ring that's there, but probably need to polish / relieve the outside of the ring to get it to be a sliding fit not a tight one);
extract and refit bearing outer race into the manual housing;
buy AR cable and gubbins;
maybe have to find different cb end cover depending on type of housing sourced. The only sort that makes remote kill/ stop button difficult is the end with the alloy dome fitting with metal button - other types used on twins have the facility for a wire to go to 'bars. Even one of them can be converted to use a brush and spring in an insulated sleeve, with a wire off.
This can all be done with the mag in situ on the bike. But if it's coming off for a general checkover, then all well and good and a complete new bearing could be fitted at the same time probably, with the armature out.
With the manual set-up on an A, you might need to change the front HT pick-up if it fouls the AR cable entry. The usual 45°-angled pick-up (if it's got one of them) isn't deeply-enough cranked for clearance - a vertical one will go on, just, and there are cranked ones available, though their quality isn't always too special.
Then you need your fixed gear - there were some neat alloy ones out there at one point (SRM I think), probably still are - and a non-Frankenstein nut for the tacho drive.
The most expensive thing there would be a replacement manual camring if you went for new, so I'd mod the old one if it's any good. Dremel or similar fine grinder and steady hands or bench fixture to enlarge the notch that limits movement, and then to make the V slot for the cable plunger. They do have to be in the right spots, or it's TROUBLE. Elongate the notch that's already there to about 10-11mm long to provide retard only - ie to allow the ring to rotate clockwise from where it currently sits. Then set the ring to the middle position of its new-found movement and mark it at mid-point of the range of movement of the plunger. Cut V slot. That way, the plunger won't slip out of engagement at full A or full R. The eccentric pin can then be used for fine tuning if necessary - to get the points to open just after the relevant flip point of the armature at full advance.
