I saw a how-to several years ago about modding a 23C to do 4x5, may even have been here.
As I recall the major work was enlarging the opening in the negative stage to fit a 4x5 neg/carrier. It also needed some lamp house mods, but I don't remember the details.
I expect the easiest thing for illumination would be to build an LED diffusion lamp house with blue and green LED's, and a controller of some sort.
That said, it would probably be easier to mod a 4x5 enlarger to be a little smaller than it would be to make a 23C take bigger negs. Mounting an enlarger like an Omega on a smaller base board or eliminating the baseboard would be easy, and the column could be shortened if there is a height problem.