Link to Flying Buffalo, Inc.'s MSPE page-

Mercenaries, Spies & Private Eyes RPG and Accessories

Porphyry works very well with the Stranger in a Strange Land, and Lost World sorts of adventures, where humans of our Earth discover a means of transport (one-way is most likely) to the blasted world.  The revised skill system in Porphyry may be more to Old School sensibilities, and MSPE characters can add the Professions without complication or omitting their individual skills.  Conversely, using MSPE as the pre-Burn world makes time-travel adventures much more accessible, and a likely welcome diversion from the horrors of Porphyry.  Just imagine taking Porphyry natives on the Raid of Rajallapor, or the surprise of the guests of Stormhaven to entertain Beastfolk.