Hey James, I found the table for Merc condensers and it shows you need a cap with a rating of .15-.19 uF for your mag. If you can’t find an original, Digi-Key has modern film caps which can be adapted to outboard magneto systems.
Here’s a link to the .15 uF ones.
https://www.digikey.ca/products/en/capacitors/film-capacitors/62?k=bc2608-nd
