I've been using
Michael Hampton for custom work (to make sure he can buy enough groceries to keep working on "
Bad Behavior", his pet project) though certainly there are many others out there, and probably findable through the ZenCart forum. Truthfully, I doubt you'll need much of anything in the way of changes, Zen is pretty complete, and there are a lot of extensions available for it. And of course, there are services (like mine) that provide custom skins for these programs.