Illinois does not register YOM plates, so you could have a different number front/rear. I don't think many would notice. Buying one plate is way cheaper than a matched pair. You are only required to carry your correct Illinois antique plate & sticker in the vehicle to show if requested. An interesting loophole in the Illinois law: it does not specify Illinois year of manufacture plates. I run a Colorado plate on the front (makes good conversation) and a YOM Illinois on the back.
If the plate has sentimental value and you absolutely must have a duplicate, it can be done two ways: 1) A cast is made of the BACK side in hard urethane, then a sheet of thin stamping stock, a 1/2" thick fiber ply rubber sheet, and heavy steel plate are put on top and hydraulically pressed. 2) If a urethane cast is made of the front side, it can be used as a mold to lay up a fiberglass copy. Less trouble to find another plate or pair unless you need that specific number.