It would sure help to have a year or serial number but I will assume it's a ADI engine so depending on which 2 aren't firing, it's 1&2 or 3&4 it's the stator but if it's 1&3 or 2&4 it's the trigger more info can narrow it down.
I have a 50 3 cyl. It had sat for 4 years before I bought it and after I ran it for a while it also ran on 2 cyl. like yours. The problem was varnish flakes/gunk in the top carb which cut off the flow of fuel to the top cyl. Pulling the carbs and cleaning was fairly easy.
