I’ve converted my at1 over to a magneto system. This might be a stupid question but can I use the old at1 coil marked 12v? I see on eBay there are several coils advertised as at1-ct1 coils. I’ve read on another forum you can use a 12v on a 6v system but not vice versa.

Short answer 'No'. If listed or CT1 plus AT1 then it'll be a magneto AT1 like we got in NZ. Your 12 volt oil will have two wires in where the AC 6Volt will only have one wire in. Actually come to think--i alway wondered how a 12 volt DC coil would work on a magneto if you earthed one wire so then just one way in!!, The AC coil is earthed through the mount.

I’ve found the coil does work, I raced the bike Saturday and my only issues were with the carburetor

Ok, really interesting info. Did you use the original coil & earth one wire.