Actually I had completely forgotten, I built one of these around not far off 40 years ago with my stepson. From what I remember, which is not a great deal, it was a very simple build with the majority of the work being in joining the two vac formed halves together. Internally was very simple and very basic with a motor and battery mount and some electronics. I even used a Bob's Board speed controller, which I cooled with a little natural flow coil glued to the bottom of it. It was never set up for serious performance but I believe it can be by those who may want to do so.
You might find this an interesting read:
https://www.modelboatmayhem.co.uk/My_models/08%20Avanti.htm