I have been using the modern car floats leftover from fuel pump module changes
Took a gas gauge out of a model a years ago that had a Christmas tree light bulb wired to the shaft, used as a float.
The A-9313-C composition float is the same black closed cell material used in small engine floats today and is not affected by todays fuels. These are a direct replacement for the original cork float. Don't see any sense in reinventing the wheel to retrofit a brass float. We have sold thousands of these and not had any problems.
On the other hand, millions of toilets can attest to the robustness of the brass ball. (That's a joke, son.100% agree with Don. The brass floats are cute, but they depend on a single large air pocket for buoyancy. If you get one pinhole in the solder – and they regularly do – you gotta pull it out, disassemble, drain completely, dry, find the hole, fix it, reassemble. The risk/reward isn't worth it, and anyway the black foam floats are cheaper.
On the other hand, millions of toilets can attest to the robustness of the brass ball. (That's a joke, son.)