Ah, this falls under rerolling a die. You can reroll crits and fumbles just as you would any other die---although if you want to rule as the GM that crits and fumbles are immune to omen'd rerolls, that's totally okay too!
The wording for 'neutralize' felt a little unclear when I was reading Mork (is a neutralized crit fail a regular fail? does it get rerolled? can more omens be spent on a neutralized roll?) so I removed the bit about neutralizing and just left the bit about rerolling dice.