<< The frames have rotted over time. >>
<< Anyone have a suggestion? >>
Any box store will have opening and maybe non-opening windows in lots of sizes.
Pick out what you like, add a bag of mortar mix and you're in business.
Might be a little pricier, but glass block makes nice basement windows. Used
that myself when some old timers failed. There is a modest improvement in the
R-factor I think, with glass block, over old sashes, It is futzier to work with
using mortar, but after the first one you'll do OK.
<< "brick molding" made of something like "PVC" >>
Could be a possibility, but getting a strong corner joint may be hard to do.