Graupner Hydrospeed

[aweb28]

I have a Graupner Hydrospeed 3:1 drive I want to use in a build, probably a mono. I'm building it up for my nephew who's starting to take an interest in the hobby. My question is what length hull would be best for this? I don't care if it's fast or not. I tried finding specs on the Hydrospeed boat but I can't find anything with weight/length etc.
Cheers
Adam