Fly Dat,
David is right FS series are 4-stroke engines and yours must be a SF which is the side exhaust modification of the RF- rear exhaust - engines. you can see one of the first generations of the FS series in the image below.


but maybe your engine is a FS R which is an old famous 2-stroke engine. the image below refers to FS R series. and a 10X6 or 10X7 will be a good choice for your engine. I have used both of them on my old FX .40 with no problem.


In the chart below there are some options for 40 FX but I suggest to chose the proper propeller based on its performance an the engine output during flight.
another proper power choice will be the .46 FX or AX.


Ehsan