What about the outward flow or efflux? The accellerating air flowing out through the fan is what is producing the thrust, right? Without a smaller tube than the inlet air (Fan dia), where's the thrust going to come from? The calculation considers the FSA or Fan Swept Area. This takes into consideration the air coming into the fan from the inlet ports and thus, the area going out must be smaller. Just don't think you're going to see enough thrust with just the fan.
I'm not an expert by any means but I don't think just having a fan there to replace a prop will necessarily work without the proper intake and outflow. A nice nacelle with a big intake and enough of a taper in the tube for outflow pressure might be the ticket. I have an eflight 66mm fan motor and nacelle combo from the eflight airliner you can have but it may not be the right scale for your F-20. A 120 size fan blade might lose the F-20 scale effect, no ?
Here's a link:manuals.hobbico.com/gpm/gpmg3910-3911-manual-v1_1.pdf