MVVS has reducers, and I made a couple myself. Still have two available, made from original UR17 spark plugs.
I know of some ignitions, that are/were extremely sensitive to changes to the high voltage cirquit, and are not to be messed with. The completely earth encased modern ignitions can play havoc with changes, so I am reluctant to answer unless backed up by experience. (none this moment, I fear)
Having said this, I have no idea how much tampering the MSR ICU ignition will allow.