Magdyno rotation

I can't tell you anything of use with regard the Mag, but I can recommend a few pointers with the dynamo. There is nothing mechanically specific with the dynamos rotation BUT if for any reason the connections to the Field or Armature get reversed then the direction of rotation will dictate weather it charges or not. If you know the dynamo was the reverse rotation to what you want then you will have to correct it, but if it is known as correct for your rotation then no need to fiddle. BUT again as soon as you try for a working Dynamo your first job is to polarise. I recommend you get the bike sorted and running with regard to its Magneto and all other points ie carburation, oil, even take it down the road, get the lights working. Then WORRY about Dynamo output (aoservices.co.uk for the Vreg Regulator fitting). With a lamp. THEN fit a regulator then worry about charging the battery.

The MSL1 had a ring cam set-up rather than the later MO1s which used the face cam and for which there are definitely cam plates marked 'R'.Presumably there are alternative rings for clockwise mags...It may be possible to replace the entire housing with a later face cam type...but clearance on the primary chaincase might be a problem. You would originally have had a Third Brush voltage control system (with resistor on the headlamp switch panel). I've not worked on one of those, but the later dynamos have a slightly offset brush holder that differs between anti-clock (which you need) and clockwise working in order to sit the brushes with the correct trailing edge. Both types work, but one will wear faster.

The armature is different between the ring and face cam magnetos. The points assembly on the ring cam is mounted on a taper and key similar to the twin magnetos. The face cam points assembly sits on a type of cutaway spigot As Richard suggest, there is a different ring cam for each rotation as when as points assembly.

If you have the original 3rd brush dynamo ( with built in cut-out) the third brush holder is different between rotation. I have this as standard on my BSA. Mine is original but many rewire them to become 2 brush dynamos and use a separate voltage regulator.