90 ls

hi i got a couple questions for everyone. my girl friend has a 90 integra ls with minor bolt ons, her ls tranny is gone i'm pretty sure it is just the syncros, well was just the syncros. it started grinding in third and now a year later it grinds in every gear. so i'm sure the gears are getting pretty bad by now. my question is, a friend gave me a 93 gs-r tranny and said we should put it on her car. the tranny is missing the case for the gears and there is minor rust on the gears. so is there a way to get rid of the rust? and can we put the gs-r gears in her tranny or do we need to get a case for the gs-r. my friend said all we need to do is get the gears hot tanked??? and the rust will be removed but we are working with limited income so i'd rather remove it myself. and the last question is would the gs-r gear ratio work for a ls engine her revlimiter is around 7k. thanks for any help.