Is the 1977 one the 2 cyl single carb machine with the two mixture controls on the front of the cowl, or the successor with a thru' prop exhaust? (I forget when they changed over)
Either way, changing that for a deadman switch is easy. Unless you have acres of room inside the cowl (the early ones do - which is why I asked!) Then remove the stop switch, take it to your local chandler, & find one that matches it size wise. Then fit that instead. If there is space to park a bus, just buy a switch with a fixing that fits the hole.
The stop switch on those engines basically shorts the plugs. The deadman is the same thing, in the case of the stop switch the contacts are held open circuit by a spring, and you push it closed to stop, the deadman is held closed circuit by the spring, and is held open by the clip. So electrically they should be a straight swap. Packaging it in the hood is usually the decider between one type of switch and another.
Worst case, drill the hood.