With the 2005+ LGT, the immobilizer would be the primary reason for non-fire. If you don't have the key that's matched to the ECU, then it's a non-starer, no pun intended.
Recommend you just get a stand-alone EM system and call it a day. Getting all of the other bits, i.e. encoded/matched key, BTS, etc., will make this a difficult project. Only Subaru techs, with a Subaru Select Monitor can encode/match the key to the system.
Pro dilectione Dei et patriam