While there's something to be said for writing your own scenarios, certainly it's true that not everyone has the time to do it. Still, there's a secret to running games that you need to embrace: converting an adventure from another game is not work, and it can be done on the fly from any game system to any other.
Whether you're looking for new fantasy adentures for your favorite fantasy game or you're converting from a different genre altogether (pulp games, for example, are notorious for a dearth of adventures, but you can convert just about any fantasy adventure into a pulp one with a few quick changes).
Here's the thing: the vast majority of every adventure module is text without a system. It's a story--a sequence of events taking you from point A to B to C. There's no conversion necessary, obviously, for this aspect. But what about the rules themselves? Let's start with the creatures.
Converting MonstersPresumably if you're running a game, you have the monster stats for the game you're running. All you need to do is substitute the appopriate statistics from your game, for the ones in the existing module. On its simplest face, consider the ubiquitous orc in fantasy gaming. Converting an orc in D&D to an orc in Savage Worlds is as simple as using the Savage Worlds orc stats.
What if there isn't an exact match? Hopefully you know your game and setting enough to subsitute something close. Just drop in something similiar. You know, for example, that a band of five orcs in D&D is a roughly equal challenge for a group of first level characters. Use a monster, then, that's an equivalent challenge for your characters from your game. Nobody says it has to be the same monster.
Consider, for example, changing your fantasy adventure to a pulp one. You probably don't want orcs in your lost underground city. But degenerate cave pygmies might be perfect. Use the orc stats, and just describe them as degenerate, de-evolved humans, and run with it.
So what if you want it to be the same monster? Again, you're the GM for your group, which presumably means you know your game and you know how monsters work. Another secret to running a game: most monsters only need 2 or 3 stats to run a combat. Drop in their combat values, a weapon damage code, and run with it.
This even works for rules-heavy games like Pathfinder. You know the general combat efficacy of the creature you're using--you really don't need to know every single feat and skill they possess. Just fudge some damage, attack and defense values that create a rough approximation of the creature you need, and roll with it.
Skills and ChallengesSo that's monsters. Now, what about skill or challenge checks? Again, super easy. Just substitute the check in the module for one in your game. Usually this also can be done on the fly. You may have no idea what a DC 15 check means in D&D; that really doesn't matter at all. You know how tough the check needs to be, and what skill or ability should be checked for your game to work. Set it and go.
Don't Sweat the Details
For years I have been converting Call of Cthulhu and World of Darkness scenarios for use in the WitchCraft and Buffy the Vampire Slayer games. It's a snap, and it works. I've done it for widely diverse fantasy games as well. You just have to not sweat the details.
This is a big problem that far too many gamers face. I'm not sure whether it's a factor of the type of mind that RPGs tend to attract, or the way we as designers present the rules, but gamers tend to be really, really hung up on the statistics and math.
Statistics and math are only the start of game design. The process is as much art s it is science, and the majority of games that are designed solely on a statistical basis, fall apart in actual play. Just because the dice should consistently fall a certain way doesn't mean they will. Again, it might make you feel more secure to have every single stat, but it's a waste of time and effort, and nobody else will know if you fudge a stat here and there to make the game work.
Indeed, you might find that not having every single number and bonus there frees you up to adapt and adjust in ways you never before considered. There's two approaches to game design. The first is, if it's not on your character sheet, you can't do it. The second is, if it's not expressly forbidden on your character sheet, you can (and should) try it.
Adopting the second method leads to a far more exciting, open and fun game with much less stress for the GM.
Remember, there's absolutely nothing wrong with taking the "quick and dirty" approach. The only real work it takes to convert any adventure to any game is the work you're going to put in anyway, and that's reading it in advance. Hell, I know some (really good) GMs who don't even do that much prep work. So the next time you need a fresh adventure, give it a go.