Hi Mark, welcome to Outdoorking.

I'm not clear at this point whether you are saying that your SB48 has the Magister ignition system, which used a round black bakelite object screwed to the cooling air cowl as part of the ignition system. It was described at the time as a Condenser Discharge Ignition unit. If you have one of those, I suggest you read this (rather long) thread:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=40796&page=1

If you have a Magister ignition system, the first question is whether it is the Magister unit (the round black bakelite object) which has failed, or it is just a fault in the breaker points or the associated wiring. As you will gather from the other thread, if the Magister is gone, that is the end of it: it is encapsulated in a plastic potting substance and cannot be repaired. Even less can it be replaced. If you want to continue to use your SB48, I think you will need to fit one of the conventional magneto ignition systems fitted to later SB48s. Meanwhile preserve the original ignition system however: complete Magister systems are rather rare, and should be of interest to collectors.