I am using the XLF from Penzoil would that be considered acceptable.That is the idle screw bracket? As in idle screw threads through it?
The carb body I believe is truly a white pot metal. I don't think that anything will work short of jb weld. But it will need to be grease/oil free and paint removed from area. I would build it up heavy on both sides and over the top. Rough it up good so JB sticks well.
That sludge is probably 20yrs old. Good quality oil will help keep it from forming.
I use interceptor oil from amzoil. It's made for power valve motors. It's a low smoke, high detergent oil. If you join and become a preferred customer its cheeper then yamalube. Another choice is Pennzoil full synthetic premium plus 2stroke. I don't know that it's higher in detergents but I use it in my boat motors and there is no carbon build up on the Pistons.
Being you've cleaned the oil tank now's the time to change oils. That way you have no concerns on mixing oils that's not recommend.
I will try the JB and would the original be sufficient