Where can I find documentation that state sales tax was paid in the 1980s when an aircraft was purchased?
This is a state level issue (here in the US) and you will have to contact the state in question. While aircraft are registered federally the sales tax is state imposed (and collected). If the family does not have the documentation you may have to go through some hoops to get it. Its also important to add what state you are in as that may change the answer to the question. 

Furthermore depending on how you are setting up the new partnership there are tax implications to that as well. You should talk to a lawyer about the various ways this can impact things. Unfortunately in the eyes of the state you had no stake in the plane (despite your informal ownership, the state is only worried about formal ownership).