We have two 4m seariders of probably around 20 year old mark (if not more) down at Neyland YC - they have 25hp suzuki 2-strokes on, which take them quite happily to 23knots at WOT according to my gps when I was sat next to one at speed the other day
The little silver plate does state 50hp max, and I'm not even sure if it even states a weight for the engine too. Looking at the transom, which is held up by two small 45 degree stainless struts with fly nuts holding it together at each end, I really would be concerned about the thing coming off with any more than 40 on there really
When the water has drained out, they're really lively boats even with just the 25hp units bolted on the back, great fun in tight turns, and highly monouverable (sp?). If the 75 has been on the boat you talk about for many years and it's fine with it, that would surely be a wild boat to play with
-Alex