I've used the 3M Window Weld on several motor mounts over the years. It's a great way to "refresh" and "upgrade" them at the same time. Actually, I just installed a "refreshed" passenger side mount last night. I pulled the original to find the rubber completely severed in one place.
Give the old mount a thorough cleaning (purple power, brake parts cleaner, etc.) allow to dry, then pump it full of the urethane being careful not to introduce air bubbles. Allow it to cure for at least a couple days and bam, better than new!
My rear mount is untouched however. It does seem as though a mold would be necessary. I can't wait to check out your work.