Re: How to stop a 1962 mercury 3.9 hp engine.
Merc intended for the motor to be killed by choking. If you prefer, you could add a kill switch which would short the points to ground.
You'd probably need to drill a hole in the bottom of the magneto plate, to get the wire up in there. Install a rubber grommet in the hole and run the wire thru. Make sure the wire can move thru the entire range of motion of the magneto plate, without binding or chafing.
Connect to the same terminal where the coil wire and the stainless flat spring from the insulated moving portion of the points are connected.
Drill a hole in a suitable place in the cowling (likely the Stbd side but your preference). Run one wire of the kill switch to engine ground, the other goes to the magneto.
Any generic pushbutton kill switch (like an old OMC switch) will do, or you can buy a fancy new switch with a lanyard. Your preference.
One last thought, if you set the idle speed down low enough, and make sure the choke is tight enough to properly seal the carb, you shouldn't have any trouble killing by choking.
But the kill switch provides an extra measure of safety.
Here's an inexpensive switch at the iBoats store:
Universal Kill Switch SeaDog Line 420488-1 - iboats
HTH...........ed