Finally managing to coax the carb apart exposes the full horror of how much corrosion can actually enter a carburetor. The float was split from freezing, and also made concave in one area due to the pressure of ice under it.
I've never met a float that I couldn't solder back together, and this was no exception. I can solder anything. It took a few hours of cleaning, careful straightening, and reforming. The difficulty was in the brittle nature of the inner side walls. They had to be tinned from inside and out. I don't know if it will hold up in the long term, but its water tight now.
The final product looks fairly good. I spent an entire afternoon with the top of the carburetor in the lathe truing up the mating surfaces of the flanges. The screw tabs were bent from over-tightening, and prevented the whole assembly from fitting together correctly without any air leakage.
It will be seen whether the corrosion of the thread holding the main jet (which prevented its removal) is problematic. The only issue now is to set the float level and find a packing nut for the main jet adjustment needle.