I can't recall if there are rules or guidelines when converting a player character into a monster with an XP value and proper abilities so they are not over-powered. If someone could tell me if they exist or where they are, it would be much appreciated.
In the Dungeon Master Guide there are some rules when creating NPCs, you should just pick the more important powers for this NPC when he was a PC, and make him likely to ressemble the PC. He will lose some features and powers, but you can manage to make the NPC stronger, but a PC will always be stronger than a NPC in the same level.
Check out the rules to create a NPC: Dungeon Master Guide pages 186-188.