the old round faced furnace thermostats have mercury switches inside . if you are stuck you could probably canibalize one and rig it up Inside your engine at the proper angle
Heat shrink wrap it for protection or apply a coat of silicone or… époxy ..to protect the glass tube.
If you need one for a 1956 30HP. get one for a 30 HP. They are specific to the year engine. I think it is the angle of the mounting tab on the switch that is different from year to year. If you still have yours and the wire is broke off just clean up where the wire is attached and solder the wire back on. Bill,
I ‘m up in Fremont wi fishing_live in burb in chicago
I think I may have one from a 35 hp Johnson RDS-21–1959
If there is way you can use it let me know
Maybe there is some way to figure the correct angle?
Just don’t start your engine with a lot of throttle in forward or reverse! The only reason the mercury switch is there as a safety devise, so you can’t start the engine at full throttle in forward or reverse. System check… am in in neutral, am at start position, if ok pull rope or turn key. The lawyers started that far back!