Fuel injection

That injection system works extremely well better than the L Jetronic on the early GTi.¹ Mz system is full digital, L Jetronic is analogue and even uses a discrete distributor which usually has a non working vacuum pod. 65 kw out of 1.6 compared with 75 kw out of 1.9.²
Injection on the 205 Si with single point injection is triggered by a 60-2 ring on the flywheel.³ If the tacho is reading, then the triggering is fine.⁴ The fuel pump relay is brown.⁵ Early Si has FP relay under the dash near the ECU, brown. Later cars have it over near the coolant catch tank.⁶ Those ECUs will not energise the pump relay and ignition unless they are getting a good Crank Angle sensor signal.⁷ They do prime when you turn the key though before starting but will not continue unless they get the message.⁸

MMFD G6 Monopoint Injection-Ignition Wiring Diagram

Improving fueling

If I ever owned an Si, I'd throw away the inlet manifold & ECU and fit a GTi manifold and aftermarket ECU. Add a decent cam as Graham suggested and it will be a hoot. If you do this don't throw away the original ECU as it is an important junction point for much of the cars wiring. Simply disconnect the unwanted wiring and leave the rest in place.Just means the ECU (K) light will always be on! Remove the globe.⁹

You could put a GTi inlet manifold and fuel system on. Need to change the ECU and would need to run a dissy (although you could proably leave the Marelli ECU running the ignition while the GTi one runs the fuel). Alternatviely, twin webbers would be a lot simpler; just leave the original ECU to run the ignition. You would proably need a better cam but could leave that for later on. Plenty of Weber set ups for XU engines available in the UK.¹⁰