RE: ducet fan for gas engine
After having some experience with ducted fans, I advise you not to try to improvise. It won't work and you will be very disappointed.
The best ducted fan setup uses either an O.S. .91VRDF or BVM .91 engine and appropriate fan. Far more expensive than the idea you have but thes will provide sucess and satisfaction, trying your engine with a fan will not work at all.
The available fans are designed for very high RPMs to develop the thrust needed and for engines with enough power to crank them at 22,000 to 24,000 RPMs.