Excellent surgery Professor Wojtek! Brought back memories of doing mine. I 'cheated' on the starboard spark plug access panel, and cut it out, scratch-built a new one from plastic card and strip, and positioned it in the open position.
I didn't have a problem with the fit of the main cowling, with the engine installed - I think this is another sign of the age of the moulds. I did have a slight problem, as I remember, with the saddle cowling though, getting the joint aligned properly, as there is very little joining surface, with the edges of the plastic being very thin.
Looking forward to more my friend.