* Fuel Pump Pressure and Volume: Fuel-injected systems use high-pressure pumps to deliver fuel at a precisely controlled rate. Carburetors rely on lower pressure and utilize the engine's vacuum to draw fuel. Even with a lower-pressure regulator, the fuel pump designed for fuel injection might still deliver far more fuel volume than a carburetor can handle, potentially leading to flooding or other serious issues. The pump needs to be changed as well.
* Fuel Delivery System: The entire fuel delivery system in a fuel-injected vehicle is designed for high pressure. Lines, filters, and other components aren't suitable for the low-pressure environment of a carburetor. You'd need to replace all of this as well.
* Engine Management System: The vehicle's computer controls the fuel injection system; a carbureted engine doesn't need this complex electronic control. Simply removing the fuel injection system and adding a carburetor won't resolve this fundamental incompatibility. The engine management system will likely still be trying to control fuel delivery to the (non-existent) injectors which could cause problems.
* Air Intake System: Fuel injection systems often have mass airflow sensors and other components tightly integrated with the engine management system to control the air/fuel mixture. A carburetor manages this mechanically; the computer wouldn't be able to compensate for this difference.
In short, swapping a carbureted engine into a fuel-injected vehicle is a major undertaking requiring significant modifications beyond simply changing the fuel regulator. It's far easier, safer, and less expensive to simply use a vehicle already equipped with the type of engine you desire.
