I don't believe that you'll find a non-legendary that would give you the same kind of reach and flexibility as an Archmage Antonidas. There simply is no other card in the game that gives you a fireball spell in hand.
The fireball that it gives you when you cast a spell is much more powerful than any other card you may get. If it lasts more than a round then the effect snowballs out of control. By that time in the game (turn 8-10) you could easily dish out 17 (2x Fireball + Archmage Antonidas attack) to 20 points of direct damage (2x Fireball + Archmage Antonidas attack + Frostbolt).
The only sort of viable alternatives that I've been able to find is a Ragnaros. It's a 8/8 on the board and deals 8 damage at the end of your turn, albeit randomly. If there is no answer for the Ragnaros, it's helping deal out significant damage per turn. Although he's vulnerable to Big Game Hunter.
Ysera might be another viable alternative because of all the cards it can possibly give you. It's a bit more of a defensive alternative where as Ragnaros is more aggressive.
Edit: After reviewing the cards, Jeeves may not be a bad idea either. Since you'll be lacking cards and most likely playing almost all your cards every turn in a mech mage style deck Jeeves can help you replenish your hand fairly quickly and you might be able to draw the extra few cards that you need.