Hello. Bit of an odd question: let's say I have a Mizzix of the Izmagnus on the field, and I have 5 experience counters. I want to cast Repeal on a Lotus Petal to up my storm count. Can I do this? Mizzix does not appear to be optional, and Repeal requires a specific cost for X, so it seems I'd be locked out of casting repeal on anything <cmc5 in this situation.
The value of X you choose for Repeal can be 0, 1, 2, 3, 4, or any higher whole number (C.R. 107.3b, 601.2b). Mizzix's effect doesn't restrict which spells you can cast. It only applies a reduction to the spell's total cost (C.R. 601.2f). A cost reduction reduces as much of the cost as it can (C.R. 101.3). Note also that these reductions don't affect the spell's converted mana cost, only the value you choose for X can affect it (C.R. 202.3c).
The value of X that you choose determines how much mana needs to be spent on the spell, not the other way around. Cost reducing effects apply after you've chosen X and determined the base cost. If you have a large cost reduction effect, you can choose a smaller value of X than what the cost reduction could get you for free if you want to.
Mizzix and specific X