So, the main aim is for boat, engine and trailer to weight sub £750kg
It's predominately going to be used for training so, no bow tank, no tabs, no oil tank, nothing. The 19 as it stands weighs around 275kg bare so definitely think it's do able
As you don't need a complete floor, how much weight are you hoping to save by not using plywood or powder coated aluminium chequer plate? That amount of weight could easily be found elsewhere. For example: a 5 litre oil tank instead of 10 litre? Do you run with the bow tank empty? Do you intend to run kevlar tabs, with one motor or two? Light weight battery? Light weight observer?
Don't sweat it, Gary