Have you enquired of Micro Antrieb for a gearwheel? They may be able to manufacture an unsplined replacement. If others are doing the same conversion, a readily available gear seems like the good thing to have.
http://www.kkpmo.com/
As a one-off, it ought to be possible to mount the gear on a lathe, bore out the centre to remove the splines and insert a bush.
To mount the gear, you would turn a cup that just touches the tips of the teeth, perhaps with just a tiny hint of an interference fit. This will hold the gear concentrically while the centre can be enlarged to remove the splines. The gear can be held in place during machining using cyano or even double sided tape. It's often strong enough, particularly if the material being machined is soft.
Mark