I would inspect the engine as you have it stripped anyway just because it has done 135,000 miles dosen't neccessiarly mean it's had it, especially if it's been looked after. You could just rebuild as it is if wanting to keep costs to a minimum

To go from injection to carb is easier than the other way, you would need some form of electronic ignition timing and obviously the carb and manifold itself. The ESC Hybrid is one such system that run carb engines right out the box but don't know if it would work with your sensors.

The RS2000 engine I would think should fit as that is basically the engine you have but in 16V guise, but then you would need to sort electrics to run that engine with ECU etc.

Martin