I need information to do a transmission of the engine to the propeller.I am thinking in it uses the cable of speedometer of cars,for engine of 20cc.That recommend propeller for whildting?

Most gas boats use a 1/4" diameter flex cable for coupling the engine to the propeller. You can use a cable from a weed whacker (line trimmer) or buy some from companies that supply RC boat stuff...there are many listed on this site. Most of them have the cables and collets/square drives. The collet you use will depend on your engine. If the output shaft thread is common enough you should be able to find a collet to use with it. If it's a weed whacker engine it most likely already has a square drive on it already.

Hi,
If you go to the speedometer (instrument repair companys)they usually carry 3/16 inch speedo cables and sell the same and they will make it a square drive if you wish. Just check the rotation first, most carry both and usually in stainless steel.
Hope this helps.