There's a theoretical maximum based on WOT engine RPM/gearbox ratio/prop pitch, the boat can't exceed this theoretical speed, because there's also "slip" to factor in.
You could buy, beg, steal, borrow a handheld gps & do some practical on-water tests. Not a bad thing to own anyway & can be picked up cheap as chips.
Or you could just make up a number, no-one is going to test it.
What licence do you need? ICC or are you taking it inland on the canals?
PS, your boat is only 10'9" so doesn't exceed 15'