I had a 1961 BSA SS80. ( C15 with a high conp engine) It was my second Brit bike the first was maybe 40 yrs ago and that one blew up.
There is a comprehensive range of suppliers who make parts for 1950’s 60’s Beeza’s. Companies like Draganfly can provide most parts. Plus the bay has everything too. Mine was completely rebuilt in 1991 then stored so I in essence re commissioned it. They are very easy to work on , no electronics or interlocks just battery switches coil points and a plug.
Lots of owners convert C11 & C15’s to 12V this aids starting and allows electronic ignition to be fitted.
Plus side the ride is immersive and enjoyable, they vibrate a lot so any diy fettling should be aimed at keeping things on the bike that are screwed to the bike. As above the wiring is straightforward, just make sure its not damaged or exposed.
Ive posted this video before on here , but this just sums up Brit bike ownership to me.
Just listen to the mechanical cacophony of metal on metal as it rumbles up the lanes. Especially noticable at 2:18” when it hits 50 mph ;)
https://youtu.be/bvoxb2MW_9s?si=4CQSNzL0udjhCf4X