Using a 7 segment display to show numbers 1, 2, 3 and 4 with four inputs
If the drivers for the original LEDs can supply enough current you can do this with a diode matrix: route the current from each of the original LEDs to the segments you want to illuminate when that LED would have been on.

In my experience a high-brightness display will be OK with ~ 1 mA per segment. Details will depend on exactly how the current LEDs are wired (to ground, to power; can you get access to the connections behind the resistors). 

In general, my first impulse is to throw a micro-controller at any problem I see. But it in this case that seems ridiculous (especially given the uC-hostile environment).