In High Moon, Ed took the Raid/Defend rules and just modified them to use for a Bank Robbery. I tried it a couple of times, with monsters robbing banks, and it worked pretty well. Since most 5150 titles use Raids as well, you can just modify those to achieve your goal.
For games in which you're actually planning a heist and gaining information, Future Tales could work. It's set-up is similar to PI in that, instead of playing one individual encounter, you're going from scene to scene collecting clues. For performing a heist, you can play a 'Get the Item' story. Scenes can include scoping the location, questioning guards or those who spend their time in the vicinity, and gathering the tools needed to pull it off.
Instead of facing a Big Bad, the final Scene could just be you're actually pulling off the theft. I'd make one small change where each PEF is either a (1-2) Physical Challenge (rappelling through a sky-light), (3-4) Mental Challenge (sneaking past a guards, drilling a safe), or (5-6) People Challenge (bluffing or distracting a guard, using a disguise). You can also use the Big Bads for rival thieves, detectives determined to bring your little group to justice, or or maybe a corrupt CEO that needs to be knocked down a peg or two.
Just a couple of ideas that hopefully help.