Hi, Lech.
The problems I encountered prior to 'The Big Crash' were probably a combination of my own assembly errors and the space limitations within the cowling. To start with, I did not get the front and rear discs of the motor mount aligned exactly parallel. Next, not all of the cylinders wound up at perfect right angles to the engine block. These small variations in alignment led to a problem when mounting the rear cowling -- since the cylinders and motor mount discs were not all parallel, the cowling could be made to align with the fuselage at the rear or the cylinders at the front, but not with both at the same time. I chose to align the cowling with the fuselage; this left some of the cylinders protruding too far forward to fit the front cowling properly (this was the fit I was testing when I dropped the model ).
There is very little room for error inside the cramped cowling. It appears that extra attention to spacing and alignment is necessary at each step of the engine construction process in order to assure that everything will fit in the cowling's limited space. This is why I am leaning towards rebuilding the entire engine. But I will have less time to work on it from now on, since I am a teacher and our school year is about to get started -- no more summer free time.
Cheers!