Waste of time to check spark that way !!!-----Make a spark tester.---Piece of carboard or plywood.----Glue wire on it with 1/4" gap between them.----Then clip on end to the block and one end into sparkplug boot.----Simple as simple can be.
Looks like a 1965 or 1966.----Do a complete evaluation of this engine before spending any beer tokens.----Do a complete evaluation ( mystery repair on transom ? ) of the " free " boat.----Why do you want to start with a carburetor kit ???
??----The motor needs oil to live !----Why would you try running with less oil ?-----Just another example of folks not understanding the concept of 2 strokes and lubrication.----What trouble shooting has been done ?
Remove lower unit.----Take it apart.------Evaluate all parts.------Take note of driveshaft and bushing above the pinion.-----Do compression test and post the numbers.----Pull flywheel and inspect ignition coils.
Overheating ?-----It does sound like it is running on 2 of 3 cylinders.----Post compression numbers here.----Check spark.----Check flywheel key.----Does primer bulb go hard ?-----Running with VRO in service ? ----Tested VRO oil output ?
That means you have an electrical issue in wiring harness or detective shift switch.-----The new switch is pricy .----Take control box apart and inspect.-----Your location ?