I’m two years late, but I like the way Ironsworn handles these “critical” rolls: when you roll doubles on the dice, something unexpected happens. A Strong Hit with a match on the dice is like a critical success, and a Fail with a match is like a critical failure. Your odds of rolling a critical in general are lower though. If you want the same odds, maybe you could designate one of your dice as the “critical die;” say if the first (or leftmost) die ever rolls a 20 or 1, you count the result as a critical/automatic Strong Hit or fumble/automatic Fail.
Happy playing!