Small update on the status of this project:
It looks like OriginalName won't be able to help after all. He wrote me a while ago that he would finish checking the script by the end of August, but I haven't heard from him since then. However, the wife of a friend of mine studied English and is willing to help me out. She doesn't speak Japanese though, so she won't be able to check my text for translation mistakes. Still, since the Japanese is rather simple and I'm not going for literal translation anyway, I think it's fine.
Meanwhile I cleaned up all the pictures that had text overlayed by hand. This was pretty tedious, but I think the result was worth it. Currently I'm looking for a font that I can use for the attached text. Maybe someone here knows a similar font that I can use?