It's not the end of the world, but also not a good sign the plate is gone. It probably makes sense to be sure you can legally own the bike before you put a ton of work/sweat/money into it. I don't know what the rules are in Ohio, but here in NYS it's only the frame number that counts with the DMV. Try to get the frame numer ran through to see if it's clean (not reported stolen or currently registered). If you know any cops, they can run it, or maybe the DMV will do it direct ("I'm THINKING of buying a bike, honest...").
If it's clean, then check with the DMV if a Bill of Sale is sufficient or do you need a God's honest Title (NYS answer for a 1977: Yes, you need a title or the DMV will make your life terribly, terribly miserable). If you need a title and only have a bill of sale, you can either brave the DMV (I think I'd rather have a root canal) or use a title service (ex:
http://www.its-titles.com./ ).
Best of luck, it looks like a good find!