When you look for a magneto to go on a Vincent you´d like one with good spark at kicking speeds , well, typically Vincenteers are quite a bit past their youth. So with bad knees this is a fact you´d have to accept and carefully pick your ignition. In the Horex community with 400 cc singles mostly they have lots of VAPE sets but some have switched back to battery coil ignition as Powerdynamo sparks are not very impressive at kicking speeds. These sets are the complete job with new flywheel magnets plus 150 W alternator and electronic ignition blackbox at € 600.- or so.
The Marelli mag is allright on Guzzi or Gilera singles with ATD on contact breaker end. So when needing to change rotation you have to find a suitable ATD for Vincent use. Plus the coil is only single spark and not strong enough for twin sparks. There are a few modern mags with rotating magnets , not coils, but I don´t think you can hide them under the mag cowl on a Vincent, too high with coil on top.
Best concept I see is a strong alternator to feed an electronic coil ignition in a distributor, cheap to do and great sparks at kicking mode. For alternators I picked brushless servo motors to go in the old dynamo place, way above 200 W 14V at 50-60 € plus cheap voltage converter. Unless you don´t care about having the mag cowl to hide an alternative mag at the old place . . .
Vic