The video shows the magneto being bench tested by a cordless drill. There was a good spark but ity was hard to start when cold and almost immposible to start again when hot. This is sympomatic of a duff capacitor or the armature winding insulation breaking down. I had the mag reconditioned by Tony Cooper from Halesowen UK who did a great job. He always replaces the armature, capacitor and slipring as these are the main parts that cause trouble.
Even with the recon mag starting was still hit and miss. I found that the BTH autoadvance unit was worn out. When timing was set to 37 degrees BTDC at full advance it was firing just after TDC on full retard. Biting the bullet I got a new unit from Tony Cooper which was the Lucas design. This sorted the timing. See the carburetor page to see how the starting issue was finally sorted out.