For budget version:
Fel Reavers are interchangeable with Clockwork Knights. You can play 2/0 (FR/CK), 1/1 or 0/2.
Archmage Antonidas is a secondary win condition, but it’s also not necessary. You can probably play a Loatheb or a second Unstable Portal if you don’t have it.
Dr. Boom can’t really be replaced if you want the deck to be successful. That should be the first Legend to craft. If you don’t have it but you still want to play Mech Mage, just like above, Loatheb or second Unstable Portal.