OK, hate to be the bearer of bad news but refastening the bottom is very labor intensive, dirty and costly work. Yes you can do it yourself, but it takes a long time, laying on your back underneath the boat. I know of some people who did it sections at a time over several years.
If it hasn't been done, you could loose a plank or planks especially on a boat that old.
There are 2 ways to do it. The quick way is to put in new screws next to all the old ones. If you do it that way, you have to be really careful that all the holes you drill are being drilled into ribs. It's easy to miss ribs and drill into nothing.
The best way to do it, is pull all the wood plugs that cover the screws, remove the old screws and replace with new and replug.
I had an old salt who did the work for me. He charged me $2000 for the job and another $1400 for the screws plus the cost of haul-out and chalking the seams. Seems like the whole job was around $5000 and that was 12 years ago.
When we did the work, the guy showed me some of the screws he pulled out. The ones fairly close to the waterline weren't too bad, but the deeper down the hull he went, the worse they got. Basically, most if not all the threads had corroded away, so there was little to grab the ribs to hold the planks in place. And this was on a boat that was about 26 years old. That's why I said, considering the age of your boat, I hope it was done some years ago. Your boat is 40 years old. It should have been refastened at least 10 years ago.
If you can't find out from the previous owners, you need to find someone who does this kind of work and pull some screws to check their condition. Pull at least 10-15 from different places low on the keel.
Now if it hasn't been done and all you are going to use the boat for is a floating condo and occasionally puttering (slow speeds only) around protective waters, I wouldn't go to the expense of doing this. But if your plans are to really use this boat as it was meant to be used, refastening is a must. Your life and the lives of your loved ones depend on it.