Hi there l've got a Zodiac 285s with the six piece floor, if you were going to make a floor you would need the
joiners to hold the floor panels together, also have you got the two aluminium supports (Zodiac call them stringers) that go on each side of the floor? You need those to hold everything in place.
With regards to the engine, the transom is rated to 40kg so the weight is OK, but l would question putting a 15hp on it due to safety issues. l've got a Tohatsu 9.8 on mine and with that l can achieve 20mph one up.
The boat is fine up to about 18mph, above that it loses stability and steering response, which is probably why Zodiac don't recommend above 8hp