70hp doesn't seem low to me at all. It was a pretty unusual pontoon back then that had anything bigger than a 40 or 50 hp on it. If you wanted to do anything but putt around/cruise with it, you also had a ski boat....
When talking more power, my concern would be the transom/engine pod. Many boats this age had "adjustable" transom heights, using some hardware that most would consider pretty light for something like a 70 (they were often rated for 40hp max). Worse, is what this hardware was attached to. That can be REALLY light duty structure.
Last, this is a boat that's coming up on 35 years old. Due diligence making a re-power decision should include a really careful inspection of the motor pod and the structure it's fastened to. Original hp rating be damned....
