I run 5MJ or 6M plugs. 6M is the specified plug. Homey do’t do dat modern spark plug thing eh. I just use the antique pugs because….. well…. I can!
You can run D9’s in it, if you want to run a warmer plug the D16 or K15J will do, I’ve seen other people do that.
I’ve not heard any anybody doing the coil swap on a Chief, but the 72T and Ace conversions to either the later OMC coils or even Speedifour coils have been done any number of times.
Bill Andrulitis also recently converted a Johnson Light Twin coil to work in a Lockwood 72T.
Best,
PM T2