I need a fan that are max 6 inches in diameter with a fingerguard. It should be able to provide thrust enough to move a hovercraft with a mass of 2.5kg and apply at least 50% thrust when reversed.
Kitchen extractor fan? 50% of what?
Google has no shortage of engines and articles on hovercraft engines.
A standard Radio Control use, pusher type propeller would work, no?
Or a model aircraft ducted fan as used in simulated jet aircraft. You’re probably needing high velocity over a small area rather than low velocity over a large area. (same normalised volume rate but different dynamic characteristics)
Propeller thrust calculations can be quite complex as the prop diameter, pitch, and rpm all have to be factored in along with the power rating and rpm range of the motor. You would probably get better information from a quad specific site or a R/C site.