Thanks for that Rex and BSAreg.
Just to reinforce . . .
EasyCaps are only capacitors so they can't 'fix' defective mags if the problem lies elsewhere!
And you DO need to disconnect or remove the original condenser (as Rex also says) unless the old one has - unlikely - failed open circuit. They normally just deteriorate and become leaky so adding a second one in parallel doesn't necessarily 'fix' anything.
60K miles so far on an old AMC twin, about 50K on my A when I had it, and fewer but still, cumulatively, tens of thousands on a load of other machines, with mags, battery / coil etc, says they work OK. I have seen a few failures over the years, but 'fingers of hands'. There are a lot of the units out there. My only mag hassles in a long time have been a failed oil seal and a brush that ate a slipring (or vice versa).
I'm sure the BTH mag replacement is very good indeed, but I'm not sure who'd be able to fix it if it broke, apart from the maker. Someone sent me one to look at, and although I couldn't mend it, I could see what was wrong. Radio Spares probably had the bits for a tenner or less, but the repair cost the guy close to 300 squid. Which is close to what CB would ask for a typical full overhaul of an always-repairable instrument.
Anyway, I'm out of all that now except for helping local mates with diagnoses and hand-holding and pointing them to sources of bits and bobs or full overhauls. my old mate KenF's wife (Ken used to post on here years ago, but sadly died a decade back) and my daughter now do all that Brightspark stuff, while I sit on my fat backside, on the seat of something with an engine whenever I can.