If I understand correctly, Terranigma was only released on PAL. Since you expected players would use an adapter you added a crystal in the cartridge so the timing would be the same NTSC system?
Did you also adjust the resolution to account for the missing tiles on NTSC?
The game that I cared about most when doing the mod was originally Japanese and then only released in the UK and Australia in English. So the resolution was NTSC compatible. I think there were some PAL exclusives, but I never ran into one that had an issue. Interesting question though!
Few console games of the era used the extra resolution so just ran squished[1] and letterboxed in the middle 480 lines. Most games tied game speed to frame rate, and didn't account for the fact that PAL systems ran at 50Hz instead of 60Hz, so just ran slower as well. Console gaming in Europe sucked until things like the Dreamcast that just took the brute force option of having a 60Hz output mode (and its now moot with HD where they only use 60Hz modes).
I doubt Terranigma had any optimisation, so would run just fine on an NTSC system.
[1] The width of the picture remained the same, just less height, so distortion.
I elaborated in a sibling comment, but my guess is that, since the game originated in Japan, which also was NTSC land, it was written to also work on NTSC to begin with.
Did you also adjust the resolution to account for the missing tiles on NTSC?