Why cant you register it? It has headlights, taillights, brakelights, turnsignals, wipers, mufflers, windshield, and a horn could easily be added making it street legal. It could be registered as a homebuilt vehicle using the serial number or even an owner applied number as a VIN.
VIN=Vehicle Identification number
So any numbers attached to the vehicle chassis could be used to ID it, making it a VIN, just like a homebuilt trailer.
1991 2.3L I4
2000 3.8L V6
2008 4.6L V8