Defroster is required by law. Odometer can be replaced, record mileage reading on old one when removed/swapped. Note mileage reading on replacement if it's not Zero. (Ultimately the goal is to maintain accurate track of the total mileage). Many electronic engine computers also track total mileage and in many cases may be more accurate than the mechanical odometers.
I don't know if your bus uses a computer-controlled dash (like IH uses) or if the gauges simply work independently (like many road tractors do) but that's the route I'd be inclined to take. There are many gauge manufacturers offering many styles, so order the few you like air pressure (either a 2-in-1 or 2 separate for primary/secondary), fuel, coolant temp, oil pressure, speed, and whatever others and build a custom dash plate for them. Don't forget indicators - turn signal, high-beam, low air pressure ...