I'm restoring a 1974 R90/6. It's had a couple of upgrades over the years before I became the owner. Ignition, rear shocks, and dual plugs. Somewhere along the way, it was tapped for dual plugs, but nothing else was done. No coil, etc. Right now the bike runs just fine with spark plugs in the bottom of each cylinder hooked up to nothing. Any kind of plug I can put in there and remove the bottom spark plugs. Don't think I'll ever consider finishing the dual plug thing. Seems to be a debate as to if it improves anything.