Any micro switch common to industry would work, most are double pole single throw 1 set contacts normally closed one set normally open. Position switch on shift linkage where it can be activated in neutral or use two switches one activated forward one activated in reverse. Either setup will be wired in series between starter solenoid and key or push button switch. If running 2 switch setup wire switches in parallel so either switch will interrupt starter signal to solenoid. Single switch activated in neutral should allow signal to pass to starter whilst activated and be open whilst switch is not activated. It isn’t a correct to build set up but, it is a safety system being retrofitted, therefore I see no harm in modifications being made. Placement may be tricky not too familiar with big twin set up, but on the 22CI motors I’d set it up to activate off the magneto plate detent arm as it is only fully in by the magneto plate in neutral; reverse and froward it moves away from the power head and magneto plate.
