I have a dual plug Saito. I run OS-F plugs front and rear. I just use a glow driver on the rear. The front plug takes care of itself.
Most old style straight 120's had a air bleed carb? Some also had a carb with an adjustable midrange. There was a big brass disk on the midrange carbs that you turned slightly to angle the spray bar to adjust the midrange.
Some pictures may help determine what you have? The 120 was made for close to 20 years before they came out with the 120S with the high lift cam. There are several variations of it.
The original style 120's could be made to run reliably but they didn't have the power that the last design 120S. There are even two designs of the 120S.