With spark at the plugs, and assuming that it is strong and consistent, the next most likely suspect is fuel. 12v at the regulator means the RSR is working. But it doesn't mean that the injectors are firing.

The RSR supplies a constant 12V to the injectors. The ECU provides the ground that makes them fire. So try testing for that ground, by reversing the test - hook the pos side of a test light to a known 12 volt source, the neg side to the ground side an injector and crank the engine. If the light does not blink, you're not getting ground to the injectors and that points to the ECU. By any chance, do you have a pink label 561 ECU.

An easy test for failed fuel delivery is to spray some fuel like substance - starter fluid or carb cleaner works - directly into the throttle body. If the car starts momentarily, then dies, it's a fuel delivery problem.

If you have spark and fuel, it may be that you're not getting enough fuel. I had a similar situation a couple of years ago in and 89 740 - good spark, fuel at the rail, properly timed. Turned out to be both fuel pumps. The in tank had failed, and the primary pump wore itself out trying to make up for the missing prime provided by the in-tank.

You mention a new timing belt. Did the problem start immediately after the change, or after the plug wires were off for any other reason? Any chance you mixed up the ignition wires at the distributor? That's an easy mistake - the four straight-up posts on the cam driven cap setup are not 1-2-3-4 -- the order from left to right is 2-1-3-4.