Are you looking to use the ARC-5 or is this for show?
The real knobs for these are hen's teeth - I have exactly one and someone else will get it at the estate sale...if I don't take it with me...<grin>. However, getting something in the hole to tune the rig is not all that big of a deal.
Take a piece of plastic tubing (polyethylene, not vinyl) the OD of which will fit in the spline hole, and with an ID a bit smaller than the tuning spline. Heat the end of it in boiling water, then press it into the tuning knob hole.
Leave it there till it cools, then it should hang onto the spline and allow you to tune the receiver easily. Put a garden-variety setscrew type knob on the tubing end to dress it up a bit if you wish - I've done it both ways, and find a knob is easier to grip than the protruding end of the tubing.
Pretty it's not - but it does work.