Thank you, Roffman. I've been giving your response a good deal of thought. I think while I'm waiting for other alternatives I'll take your advice and, after letting the old float bake out at a moderately-elevated temperature for a day or so, I'll coat it with some Red-Kote gas tank sealer that I have left over from my adventures with the Beloved 1964 Econoline van. After a day or so of letting it dry I'll re-install the float and try once more. Myron's Mopeds offers a float that might work for $25.00, and I've written to them for advice along those lines as well. It's a shame that parts are so difficult to get for these bikes, for they're built like a Korean tank and are unlikely to fall apart like my ancient motorized bicycle.
I should add that a motorized bicycle is absolutely not the same as a moped. The geometry for riding and steering is incorrect, the brakes are a joke, there's no electrical system or suspension, and the whole arrangement is precarious. The idea is fun--the kit including the engine cost eighty bucks--but 25 mph on a bicycle that was never meant to be powered is a terrifying speed.