The (NGK) recommended plug for a standard Vin (Rapide, Shadow, Comet, Meteor) is an NGK BP6E, although if plug fouling is a problem, a 5 will be even better. I've run both. IMHO 7 is far too hard. (The plug for a Manx Norton is an 8.) The plugs with small centre electrodes carry the suffix "V". A plug takes less voltage to spark if the centre electrode and side electrodes are filed square with sharp edges. I suggest trying an NGK 5E before experimenting with something more expensive.
I've used NGK BP6ES without problems, in a variety of motors, for years, but stopped because I found new NGK plugs failing after a few hundred miles. This has been happening to me, and to others, for several years. There's been some discussion of it in the forum. I now use NoS Champion N9YCC (copper core), the exact equivalent of BP 6 ES.
There is no reason to use resistor plugs with a magneto (NGK resistor plugs have an R in the prefix) and good reasons for not using them: it may be true that they work if the mag is in perfect condition, but that's because if the mag is NOT in perfect condition, they will accelerate its demise. My twin wasn't an easy starter with NGK BPR plugs (they weren't labelled as resistor) until I added caps that weren't labelled "resistor" either. No more problems after that: there was no spark at all. I had to get the mag rebuilt.
My experience has been that starting problems are more often carburation... even if the mag is weak, it is at its strongest when cold, failing once it heats up.