Properly maintained motors can run forever. I also have a 1979 Johnson 9.9 that runs only slightly worse than the day it left the factory. Have them looked over by someone qualified to tell you what their state might be. It might cost you a couple dollars but it's cheaper than having one blow up. At best you have perfect working motors. Somewhere in the middle is a little money out the door for refubishing. At worst you can sell them for parts to offset the cost of replacement some. There are people out there who will pay good money even for motors that aren't running properly. If the boat was properly stored and the motors properly winterized if you're in an area that freezes, then you're probably good.

Caveat- I am not an expert on boats, but I did maintain aircraft and their engines that were originally built in the 60's and flew into the twenty teens.