Well, if they were later Cobra out drives, most of us would tell you to run for cover. Some would say that with patience and understanding, they're fine. I agree with the latter. I have two of them, have fought with them both and mostly won.
Unfortunately, the boats being from the 70s probably means Stringer drives. I think in that case, running for cover would be unanimous.
As far as twins vs. single screws, there's really no comparison when it comes to maneuverability at the dock with twins. It takes some practice, but it's pretty amazing. Having a "spare" engine in a crisis is nice too, but with this come twice the maintenance and hassle.
Myself, I'll only have twins in the future, and I will simply never have an I/O again - period, regardless of brand.