The vent portion in the tank is what I would replace right? The little rubber disk with a nipple on the top? Part no 1110 353 1600 I think
that red rubber flapper in the tank doesn't really matter if it's in or not. As long as the valve is good. if the carbs sucking petrol it'll be from a leak at the breather hose where it joins the tank, fuel gets sucked in from beneath the airbox, should be obvious if it's the case