To be honest, you won't find many knife makers that will rehandle an existing knife. You ought to give Bobby's suggestion some thought. You could get the materials for probably $20-$30 and do it yourself with a drill, a couple of files, and a couple sheets of sandpaper. Another option that would remedy the problem he mentioned about antler scales not being flat is to use jigged cow bone. You can get it dyed various colors and it would start out pretty symmetrical which would make the fitting much simpler. I also like the look of smooth polished cow bone.
Matt