The first shop I worked in had a complete TA case, and we built up TA cranks to suit all the time, yet I don't think we had one of those
#24 tools. Maybe we did; it's been a long time.
Chainring bolts freezing shut was always an issue even BITD. I quickly learned to always use generous amounts of grease on them, but not everyone did. 40+ years later, of course there will be even more hassle.
Anyway, I would search for a big screwdriver that fits the slot well, and then square up the end with a grinder or file. Soak well with your favorite penetrant beforehand, and proceed carefully.