Unfortunately, I can't just re-release someone else's work modified, but I can show you where to get the files.
http://www.flightgear.org/download/aircraft-v2-6/That link contains aircraft downloads for Flight Gear. Many of these models use the jsbsim flight dynamics model, but watch out, because some use YASim instead, which is incompatible with Outerra. The X-15 was one of the easiest planes to convert to use in Outerra.
Download the X-15 from that site, then take the X15-new.xml file from the main directory and place that in your outerra directory /jsbsim/aircraft/c172r/. Back up the original c172r.xml file and rename the new X15-new.xml file to c172r.xml. While the X-15's engine config files (the XLR-99) already come with the jsbsim version packaged with Outerra, many aircraft you may want to try will require you to pull the engine and the nozzle/prop/rotor file associated with the engine from the Engines/ folder of the downloaded aircraft and place them into the /jsbsim/engine/ folder for Outerra.
Keep in mind that the X-15 and a few other aircraft are fully compatible, but most are not. Flying other aircraft took some heavy xml editing on my part to even get them working in Outerra, as I don't believe Outerra currently reads or runs any of the systems files for aircraft and there is no way raise/lower landing gear or use any aircraft that has more than one engine. On a few aircraft, I actually had to zero out the effects of drag and pitching moment on the use of landing gear. Other aircraft, such as the P-51 use complex engine modelling which requires the use of systems files, so even after I got the P-51 running so it wouldn't crash my game, the engine was underpowered and was unable to pick up speed. Keep these things in mind if you feel like the aircraft is not behaving as it should.