Just to note - the original ski-jump on 'Invincible' was fitted alongside in about 10 weeks, and didn't require a full refit. All depends on how big a jump and how far forward they want to install one, in my view. Invincible's first ramp was limited by the Seadart launcher located centreline forward - I can't see any obvious problem for 'Izumo' or 'Kaga', apart from a Phalanx CIWS located on the flight deck right forward starboard side.
Also worth noting that the maximum effective angle for a ski-jump with F-35B is less than that required for a Harrier, so the size of the ramp should be slightly smaller. Width required is about the same as for a Harrier.
Hope this helps,
Engines