On a very remote world the defeated by combat a handful of B1'ish battle droids and a few security droids. They decided to keep the droids but they are pretty shot up. I suppose like anything else, if missing spare parts they could use a some droids for spare parts to fix the rest. How would you handle fixing up droids with no spare parts on droids that are very, very old?

Setback (maybe two) for antiquated technology / no parts. Difficulty might be increased, depending on how unfamiliar the droid model is to the PC.

How about this....A failure with threat means that it cannot be repaired without X Threat number of robot repair units . A success with threat means it was repaired, but hobbled together but still needs x units to be fully working. A scraped droid=Y robot repair units ? Each RRU=Z Credits. Or maybe it would easier to just fiat the whole thing.

Or maybe it would easier to just fiat the whole thing.

You always fiat the macguffin unless you want the story to end prematurely or stall into boredom.

What? You didn't know they picked up the macguffin? The PCs didn't know that, either. Bless their hearts, they were playing in character and snagged some extra salvage. Little did they realize that those were the droids "they" were looking for...