So for those 50-mph thrills. This Hydro-Kart project was very popular in the early 60"s. I say that because both Popular Science and Popular Mechanics published similar plans around that time. I think something like this could still be a pretty neat project. Certainly your parts sourcing will be different, but if anything, you will have a greater selection in todays market. Smaller engines and more HP for instance, could eliminate the second engine. If I remember correctly the PM plan used a small outboard, a similar size today, probably puts out twice the HP.
So from the 1962 "Science And Mechanics" magazine is a plan for a 50-mph mini-speed boat.