The prop first:

I take it the prop doesn't fit on the shaft, is that the first dilemma? If so, that's the way GWS sends them out to you. The prop shaft is 4 mm in diameter, so you need to drill out the hole in the prop to that dimension, which is very close to 5/32. Ensure your new hole is as centered as you can get it and perpendicular to the prop--you don't want the drill bit going through at an angle. Do your best. Hard to get it perfect unless you have all the right tools.

As far as the cowl not fitting, I need to see pictures of what you've done. The trimmed cowl should just slide on to the nose of the fuselage. There are some small plastic squares--maybe one scored larger square (less than 1" square)-that are glued on around the cowl that give the cowl mounting screws something to bite into. Photo 75 shows these being glued on. If you look around the edge of the trimmed end of the cowl, you'll see 4 pits. This is where the screws will go through. Drill them out before mounting the cowl.

I can't remember how much the spinner sticks out behind the prop (I don't use the spinners anymore....). So, put the prop on your motor and put the spinner on and measure that amount. Now take the spinner and prop off.

Install, but don't glue the assembled motor into the front of the fuse before doing the next steps as you want hole in the front of the cowl to be centered on the prop shaft hole

Tape the cowl to the front so the hard plastic pieces are underneath the holes and the prop shaft is centered in the the hole up front. With the cowl taped on, drill very small guide holes (you need numbered bits for this) and then screw in the screws.

If you don't have very thin bits, just screw them in. Remove the tape carefully.

After that is done, loosely install the prop. slide the stick in or out so the prop and spinner will clear the front of the cowl.

Carefully remove prop so it doesn't move the stick.

Now take the cowl off, and mark where stick enters the fuse. This is the mark you will use when you take out the stick, add glue and reinsert it.

I hope that's clear--had to do it in a hurry....

Working on these foam planes is sometimes easier if you have a stand. I have a box with two large Vs cut in them that are covered with foam. I can rest the fuse on it right side up or upside down. You could make something like this out of big blocks of foam glued to the same piece of foam or cardboard so they don't easily fall over.