It's seriously doubtful that there will ever be an accurate model of this car.
There was only one car ever made, no blueprints were ever released, and it's highly unlikely that the owner will ever let any videogame modelers get their hands on it, especially the Forza 3 creators as their method of getting cars into the game is considered to be "cheating" by any serious 3D modeler who can make cars from scratch.
The Forza guys need to have the car physically in their hands so they can "prep the car for modeling" by taping a grid onto half of the car with 1/8th inch masking tape, then they scan the car into a program with a 5 Axis Laser Scanner, once the half-model is scanned, they just mirror the other half. There was a pic linked on their forums of a Mitsu Starion being prepped that way.
Considering how many glaring realism mistakes the Forza team have made over all 3 games, if I were Glickenhaus (the owner of the P4/5), I would never let them touch the car.