You can use a 2 bar map sensor from a Weber-Marelli (if still available) or any other major manufacturer (like Bosch) that produces a 0-5V signal (most of them do) in combination with a aftermarket programmable ecu. I would strongly recommend you to use one in this case.
In this case you also don't need to fool the original map sensor as you would not be using it at all.
In this case you have complete control over ignition timing also means different timing for different loads.
You don't have to retard the ignition moment very much for only 0.1 bar of boost, whereas you would retard it more for say 0.7 bar of boost.
So, ther'es absolute no need for grounding the timing pins on the ecu.
The same goes for fueling, you would have complete control over fueling with all possible loads.
So, with bigger injectors you don't need the 5th injector.

You cannot use any 2 bar map sensor in combination with the original ecu, because this will cause under fueling over the complete range, including off boost driving. Not to mention that you cannot use the Cosworth 2 bar map sensor or any other map sensor with a 0-5 volt signal in combination with the original ecu because the ecu would not recognize it's signal at all.
Grounding the timing pins on the original ecu is ONLY done when driving on boost, otherwise you would ruin the fuel economy off boost and also loose power when driving off boost. So, you cannot permanently ground these timing pins. That's what they use the 2nd map sensor for.

With a aftermarket ecu you can also control the turbo boost if you would like to and various other things like cooling fans.
These capabilities have a positive effect on safety and durability of the engine.

You are probably not capable of programming the aftermarket ecu yourself.
So, I would suggest use the one the tuner that will do the mapping recommends.