The farther back you go in time for about 350 years, the easier to be a polymath. In the 18th Century, Franklin, Kant, and Goethe could make sizable scientific contributions in their spare time. In the 17th Century, polymathic geniuses were thick on the ground, such as Descartes, Leibniz, and Pascal.
What 20th Century figure would have cut the widest swathe in the late 17th Century? Off hand, I'd guess John von Neumann, the mathematician-physicist who played such a large role in the Cold War despite dying at only 53 in 1957. Considering he made a deathbed conversion to Catholicism, I could imagine him converting earlier in the old Austrian empire and rising to be Prime Minister as well as a towering figure of Newtonian proportions in the sciences.