Hey, so I have extracted the Castlevania 2 ROM from the GOG version .exe, using the instructions/script provided in (another recently-active thread). It works great and the game runs well in Nestopia. However, I had hoped to use the re-translation rom hack provided by bisqwit. I downloaded the patch and applied it (using bspatch), but when I try to run it I just get a green screen in the emulator. Has anyone managed to successfully apply this patch, or have any idea how it could be made to work? Perhaps it is just expecting too much for a rom hack patch to work on an extracted ROM file.

This patch seems like it would be very useful, since the original English translation for Simon's Quest is notoriously poor and confusing. It apparently also includes other nice QoL features, like a built-in map, support for save slots, improved day/night cycle transitions. It would be great to get it working with the GOG version, if possible.

(Sorry, but unfortunately I don't seem to be able to post links right now.)

Well, this ROM-hack is clearly targets a specific ROM image with specific hashes. Check project page at RHDN for actual values: https://www.romhacking.net/hacks/1032/
If you extracted ROM image has different hash values then it's unsuitable for the hack.
But if hashes are the same then you, probably, should try a different emulator.
I only tested re-translation hack on the real hardware, though. And on the NES mini.

--- Update ---

Yes. Hashes are different, Extracted ROM image is NOT suitable for re-translation.
If you still want to go this way, here is a diff which should be applied with xdelta3. It will turn your extracted ROM into a suitable one with proper hashes and contents.
https://zippyshare.day/Ye3FPXvG9yGIfq2/file

Hi, thanks very much for doing that and sorry for the slow reply. I've suddenly gotten really busy with some work stuff, but I will try this out over the next few days and let you know if it works.