how do you know about petrol pissing out the manifold? surely you have an exhaust on the bike!
as for the dying thing, yes this could well be an indication the seal is gone, in effect you maybe starving the motor of the air it need to burn with the fuel, and so it dies. Probably the carb throttle adjustment is turned to fully shut the throttle because of the fast idle caused by the seal leak and so when the WD40 plugs the leak...voilla it dies!
Can be changed easily with engine in the frame, you will need a flywhheel puller...i think they are usually 27mm (check) I understand if two small holes are drilled into the seal at opposite sides and then self tapping screws are threaded into the holes....This gives something to grasp with molegrips and pull out the seal, best done when the engine is warm, maybe use a hot air gun to warm the case around the seal area