One of the first substitutes for lead filler about 40 years ago or more was a product made by Ditzler, I believe, called "Aluma-Lead". It was a metallic powder intended to be mixed with clear lacquer into a thick paste, and applied with a putty knife, then finished like Bondo. I don't think it's still being made, but a suitable substitute would be a 2-part 3-M product called "Epoxy Steel", or another metallic filler by the same company known as "Epoxy Aluminum". I've used both products to reform places like rotted window channels where a more durable fix than Bondo was required.
Jerry
