While I sympathise with our need for a dense, cheap easily workable/available ballast, with my ex-analytical chemist's hat on may I add a word of caution?
Lead is a toxic heavy metal known to interfere with the bodily processes, organs and nervous system development of those who are exposed to it (eg by skin contact, ingestion of contaminated food or tobacco use etc.), leading to the long-term development of learning and behavioural/cognitive disorders, particularly in the young.As with many toxins, the development of symptoms will be dependent on the duration and intensity of one's exposure to materials containing it, old lead-containing paint and paint chips being a common hazard, as are our beloved lead fishing weights which have poisoned and killed off so many of our swans.
With the above in mind, the wearing and subsequent disposal of a few pairs of latex/surgeons gloves whilst handling it would be a vital and effective first precaution if one is still prepared to take the risk of using it, and its disposal as a special waste to eg the scrap merchant when a model containing it has reached the end of its life would be the responsible thing to do.
But what to replace it with? Copper is quite dense with a Specific Gravity of 8.9, not as good as lead at 11.34 I know, brass weighs in at 8.38, with steel or iron at about 7.8ish.Or for the financially unchallenged, there's gold at a hefty 19.3...must go that will be the chaps from the Fishing Weight Marketing Board at the door, come to make me an offer I can't refuse!