Hello Jack,
that's a real beauty of a Stuart! Exquisite detail!
Your headlights turned out a lot better than mine usually do. Some people think drilling them out is the only thing to do. For filling the "glass" part you could also use white glue, the kind that dries transparent.
I think painting the lenses is avery practical option. Once I accidentally got very nice results by painting the lens silver (that was intended as base for white), and then putting particularily badly covering white on top of that (you might want to substitute thinned down white here). Then I looked at the headlights and I said - done! Who would have thought? Here's the model I'm talking about:
Good luck with your builds and have a nice day
Paweł