Well, Magnum uses bad Chinese-made Carburator and leaking is there for sure.

Anyway to me seems the best to stop any 2S engine is to open the crankshaft case. That is simple
by placing a valve on niple which is usually used to control an optional Perry-pump. Check the
upper-left bolt of the back plate of the Magnum if it has hole which is opened to the inner space
of the crankshaft case and then instead of the bolt place a niple. Alternatively, drill the back plate
and mount niple there. One more spot is on the crankshaft case right on the bottom, opposite to
the carb; there most of the old engines took pressure for the fuel tank. This one is even more
efficient than the back plate. In any case if you decompress the inner space of the engine it
stops very fast.