Doug,
I guess my question is why you'd want a more modern set-up. If the answer is that the current combination is unreliable or damaged, then a replacement is an easy thing, for well under $200. If you are looking to retain a sort of vintage look to the images, the planatograph is probably as good a lens as you need. While not as sharp as a modern lens overall, it has a soft/sharp quality that works well for postcard sized images and can be very sharp in the center of the image, stopped down. It is also convertable, allowing you to shoot two or three different focal lengths with one lens (although the image quality does change some with only one element.) For a simple and inexpensive change, you might think of putting on one of the many tessar type lenses that are available from large folders and plate cameras. I know a 135mm Zeiss or B&L tessar in good shape certainly shouldn't set you back more than $100 and it has a different look to it than the RR style planatograph. A modern, coated lens will be more expensive, but they're out there in the focal lengths you need in your price range - just keep looking.