There used to be a lot of common ground (at least in the UK and Europe) between older Mitsubishis in the area of braking systems. Rather than go all-out with Brembos, perhaps a better solution would be a set of stock twin-piston calipers (and bigger discs) from another Mitsubishi (EG a Galant 2.5V6 or -admittedly guessing here- a set from an Eclipse GSX) ? Given the light weight of the Mirage, I'd imagine this would be more than enough, and I'd think it'd be easier to get hold of spares for offering up, etc.
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View my fuel log 2014 Mirage 3 CVT 1.2 automatic: 47.9 mpg (US) ... 20.4 km/L ... 4.9 L/100 km ... 57.5 mpg (Imp)