I've completed the first version of a Monster as Player character system. It uses the stat block to generate "Racial Modifiers" for the Ability Scores and the traits for a "Class".
By translating the creature CR to a Level Equivalent, the system pads the Character Level to bring the power level of the creature in line with that of an official player race. I've used the NPC table to help map CR to a Character Level Equivalent.
Please look it over and give some feedback.