I've just done the first run-thru of the May Good Old Boat articles and one is about exactly this problem on a 14-year-old Yanmar.
Long story short, the author simply ran two new wires from the key switch to the starter solenoid . . . problem solved.
He has the 3-meter harness (Yanmar apparently also supplies a 6-meter harness) and he used 8 AWG wire, tho he says 12 AWG would probably suffice.
Ok y'all, now go buy your subscriptions . . can't keep giving away the material here!