Earlier this year I built up four modified MO1 Mags from parts with 'Brightspark' condensers - They produce good sparks.
The trick is to remove the original condenser and replace the coil connections with something set in an infill medium. I experimented with 'Clearcast Plastic' (too brittle), a machined nylon block (too difficult to machine to a good fit) and finally epoxy resin (preferred). I learned a lot.
Regardless of my OCD experimentation the actual 'Brightspark' condensers are good
(I wouldn't recommend building a mag from disparate parts - the bits don't fit together well or run true and there are a surprising amount of design changes over the years. My lathe & mill had a lot of work to do).
My avatar shows the late Len Rich in 1970 with the bike I now have - a 1958 Ariel VH