Many thanks for your supportive comments, Mike, Stephen, Rick and Andreas, they are highly appreciated!
While I was out of home during this week, I was thinking about possible solutions for the opening in the engine cowling.
In the first step I will test out how the decal solution will look, but plan B never goes amiss, right?
So the plan B will be a cylinder made of brass I will insert the cowling into. The bottom side of the cylinder will have openings identical with the ones I want on the cowling and thus serve as a guide while drilling out the holes.
Will see, how it will work.

Cheers
Ondra