I think RCJI will give u the solution. I'll have a look at my issues, I think I have it.

However, I've seen LOTS of RC hovercraft working on std .25 aircraft engines (not as powerful as yours) with std props (sometimes cut) and they seemed to work great.

What is your maximum fan diameter ? If diameter is really an issue, then yes you are bound to use a little fan, which seems difficult to me, but feasible.