Aside from the legal aspects of it [kill tag etc] all you need is a cape that is close to the same size as the original. If you wanted the mount to look like the original, measurements from the original would be helpful to the taxidermist. Neck size, corner of eye to tip of nose, antler base to tip of nose, antler tip to base of nose, etc. But any "good" taxidermist can remount an old set of antlers on a new cape easily.
I've had customers come into my shop with a cape off of a 1.5 year old and wanted me to put a 5.5 year old set of antlers on it. It can be done but it isnt going to look realistic. A taxidermist can stretch the cape somewhat to fit a larger form but only so much.