RR's main problem now is engine production to make up for the lack of spares.
Which may not be as difficult as it sounds.
As I understand it the engines are leased by RR on a "Power by the hour" basis with a number of spare engines always available at each customer's base.
The latest production engines are believed to already have the required modification so the problem can be simplified.
1) How long can the 40 existing engines remain in safe service given regular (daily ?) inspections.
2) How long does it take to change, strip, modify and replace each engine.
3) Are adequate supplies of modification kits available and if not when will they be available.
From this it is possible to determine how many more spare engines are required to keep the fleet in the air while modifications are carried out, the shortage is probably rather less than 40.
Just my 5p worth, I have no inside knowledge, just trying to inject some sanity into the speculation.