Franco....

....you have to plan your cuts. Look at some other planes and see how they are done.
You will have to cut your plane by yourself. This is the learning process, and it is fun.
Don't be afraid to make a mistake. You would do well to invest in a Dremel tool, or
something simular if you are going to hobby.

First you must cut the opening so the engine can fit in. Then you must cut for the muffler to fit.
Finally, you must cut for the needle valve access, and anythig else that needs trimmed.

Here are some ideas on cuts....[sm=thumbup.gif]....just go for it.

FBD.