Power conversion

I have been running HO for many years, but recently had to scale down to a move to a much smaller living place.
SO... I went all in and embraced Z Scale.

Question is: As I have many old 12v power controllers, and I am not ready to convert to digital yet, I would like to use one of my 12v controllers to power my Z scale. I am aware that z scale should be only 8-10v, so what I want to know, is how hard is it to convert or change an old controller? Is there a resister or step down module i can buy? Any suggestions?