* Block Design: The SOHC block is not designed to accommodate DOHC valve train components (camshafts, rocker arms, etc.). The cylinder head mounting surface, the placement of camshaft journals (if any exist in the block), and overall architecture are entirely different.
* Cost Prohibitive: The cost of engineering, machining, and fabricating the necessary parts to even attempt this would vastly exceed the cost of simply replacing the blown engine with another compatible used engine or a remanufactured one.
* Lack of Parts: There are simply no readily available parts to make this conversion. You'd be creating custom components from scratch.
In short, focusing on finding a replacement 1.9L SOHC engine or a compatible DOHC engine from another vehicle (though that would require a much more extensive swap) is the only sensible approach.
