depends on what type of clutch and sprocket you use. If you buy a 35 clutch and sprocket you will buy a 35 chain. If you buy a 41 clutch and sprocket you will use 41 chain
It all depends on the sprockets. If you have a sprocket on the engine already, measure the distance from tip to tip (called the pitch), and then look at this site to figure out your chain pitch and #