Here's an older mercury switchbox for your 2-stroke, 2-cylinder. Worked when it came off the motor and was kept as a spare, just in case. Sold the boat and the motor, now have just this left.
Part number 338-4733A2, came off a Mercury 402 40hp. Retails for $250 off OldMercs.com
Fits the following motors:
BRAND HP YEAR MODEL SERIAL RANGE
Mercury/Mariner 20 73-77 200 3537531-4709592
Mercury/Mariner 40 72-74 402 3336258-4117709 Manual Start
Mercury/Mariner 40 75-77 402 4117710-4860102 Elec Start
Mercury/Mariner 40 75-81 Man. Start 4117710-5823917
Mercury/Mariner 40 78-81 Elect .Start 4860103 - 5823917
Sell for $150 (offers?)