American Pewterers where making leaded pewter in the this country into the late 1700's. Thats what was available. And I agree that the best pewter of the age was nearly all tin and copper. Still antimony that has been added won't help them brain cells either.
What is the issue here fix it or don't, and buy new. If its old and valuable, buy new. If it's really old and heavy with lead, its valuable, buy new. If its new and it leaks, it isn't very valuable. Buy new.