the 270/300 twins use the old Saito 120 carb on the single carb versions. Being as the cylinders are alternate firing, the carb the correct size for one cylinder is big enough as only one cylinder fires at a time. So the carb for the FG36 may be a little large???

The FG30 carb may be better?

On a twin ignition set up, there is only one trigger point and it simultaneously fires both plugs. Therefore each cylinder is firing an extra time during the overlap stroke which doesn't hurt anything as there is nothing to combust in the cylinder.

The single cylinder four strokes also fire every time the piston comes up. Most cars do this also now days. I had a 1987 Cavalier 4 cylinder with two dual outlet ignition coils and no distributor.

Try your 270T with the dual outlet ignition and the original carb on gasoline first and see how it runs before trying a WYD Walbro. You could also use a WT-457 Walbro from a Ryobi four stroke?