IMO I think that you should first consider doing something other than a traditional mohawk, orks not having any hair. A nice row of metal studs or spikes would look cool and be easy to acomplish with a piece of brass rod, a file and a pin-vice. Or perhaps a dorsal fin ala the Savage Dragon comic book hero. To do a regular mohawk you would probably best off making a thin ridge of putty down the center of the head, waiting for it to cure, then putting on a second layer into which you could scribe the hair. Your first layer gives you the basic shape and lends stability to the second layer. If you have it I suggest you use miliput to build your ridge. It is far more durable and less flexible than kneadatite, that will give you a stronger surface on which to work when making the hair.
Good luck.