What model multicam is it? I have rewired a few older multicam's and they have all had transmissions on both sides. The big issue with switching out motors is keeping the same size to fit the location and I have not seen servo's with encoders that are dual shaft. My experience is the motors are normally good and I change to modern drivers. I have a 48 plus M cam about finished and I did upgrade to new motors, but stayed with steppers.
It might be possible to mount a motor outside of that shaft that runs to both sides and belt drive that shaft. Build a motor replacement section with a shaft and gear mounted on it. Having the space to do that may be an issue.