So I got a Tohatsu MFS15C and after studying the parts list, the only difference I can find between the 15 and 20 hp is swapping the following parts in the carb:
"main jet" to 3BJ-04421-0
"plug" to 3V2-03169-0
"slow jet" to 3BJ-04552-0
"main nozzle" to 3BJ-03141-0
However, there was an old...