can the keybed of the cp4 be installed in the Montage?
No.
One method of installing the CP4 keyboard "with" the Montage is buy a CP4 and connect the CP4 to the Montage through MIDI. Using a CP4 as an external controller will give you the keybed of the CP4 if you prefer that and also will give you a different piano engine which uses modeled technology (SCM - Spectral Component Modeling - which is Physical Modeling). This is different from Montage since Montage doesn't use SCM.
Current Yamaha Synthesizers: Montage Classic 7, Motif XF6, S90XS, MO6, EX5R
What would it be like to change the keybed of the Montage 8?. If you can not play what you want with the Montage 8 KB, it's more up to you to perfect your technique 😉