RE: Cutting Fiberglass Cowl
IMO the best method is to tape a piece of light cardboard, e.g. from a manila folder or similar, and with the engine in place, tape the card board to the side of the fuselage so it covers the parts of the engine that will protrude through the cowl. Then, cut the card board to allow the engine to extend through the cowl as needed. Finally, transfer the cardboard opening to the cowl and cut as needed.
There have been posts on this subject quite recently; I would recommend a search to find the posts which are explained in better detail than what I have provided.
DaveB