"I checked a number of places
"I checked a number of places where I usually find discontinued Merc parts and nobody has any parts of that assembly still in stock.
You can keep looking but my suggestions would be as follows:
1 - see if you can find an outboard recycler, they may have used parts (and the assembly was the same on 18, 20 and 25 Mercs of the same vintage running Walbro WMC 9, 10, 20 or 25 carbs)
2 - see if you can jury rig the assembly from an 6/8/9.9/15 model - they are fairly close in design (internally)
3 - if you carry a battery on-board, go with the electric choke (primer) which is still very much available (replaces that whole assembly) - Merc part 1395-823433 (but retails for about 50 bucks)..."