Quick update.
After doing some looking and some talking with people, 1.9" tires are going to be too big.
I decided to go with some 1.55" beadlocks, and I found some tires that are a smidge larger in diameter (about 3" vs the stock 2.76"). I will have to go with the 12mm hexes, but that's okay.
Working on a somewhat unusual build.
Have a X-Ray M18MT I want to convert to a crawler. It is a 4wd 1/18 scale truck.
I found a way to convert the unusual axles to a 12mm hex, if needed, but I may not need to.
The factory proprietary wheels may be the right size to use 1.9" tires.
The wheels...