One would have to identify a donor wheel with identical sprocket carrier and brake disk mountings.
Is the bike in question an alloy wheeled f750gs or a spoked wheeled f850gs ? If the former then search the BMW range for a chain drive bike with an 18 inch wheel. Not many about I would say.
If an f850gs, then probably lacing an 18 inch rim onto the original hub is an option. This may affect the speedometer read out and the brake abs function.