Kromlech.eu has some shots up of some new Monowheel bits that would look awesome stuck to an Ork.
These bits should be availalbe from Maxmini.eu within the next month.
I am loving these bits - you could use them for Nob Bikers, regular War Bikers, or even just to spice up a unit. Now word on the price yet but I expect them to be in line with the existing set of legs that are on offer for about 2 Euros a piece.