https://github.com/brianluft/heirloom/commit/3001b284130c399... is rather interesting. Not only for all of the implicit type conversions that the code turned out to be doing, but also for all of the things that were dropped.
It is not totally "modernized", though. Its idea of "Unicode only" is using WTF-16 rather than UTF-8 (which is possible on Win32 nowadays with code page 65001).
I always install IBM's Workplace Shell for Windows on Windows 3.x which replaces PM and FM with a shell that looks and works like the OS/2 Workplace Shell.
It's open source and the installer is available here: https://winworldpc.com/product/ibm-workplace-shell/151
Windows 10 is applying smooth scrolling to the windows, which I can disable in Performance Options but you might want to override the setting. Waiting for the scrolling to catch up is pretty at odds with the "fast, lightweight" aesthetic you're going for.
1 px window border and "modern" scrollbar (small, without end buttons). Modernized /s