So 1.6 Mantra ...
Please, it is a Matra not a Mantra!
Anyone who knows are the enginemountings on chassis side different on 1.6 and 2.2 ? I would still prefer for a 2.2 chassis.
The two chassis are different and the 2.2 is not suitable for the XU or that family of engines. You would have even more mods. to do, particularly to the chassis which then ruins the galvanising. Since the 1.6 is close to the XU family the mods. required to fit are certainly easier and possible.
However, I'm with Krede on this one, forget 300bhp in a Murena. It is far too much and will totally spoil the car even if you ever get it right. Everything will need to be uprated to the point where it is no longer a Murena. If you want that much power go buy something like a Noble or whatever. Please don't spoil a great car.
If you want 200bhp then start with a 2.2 and turbo it, a la 505 turbo, which used the same block; but beware you will have clutch and cooling problems unless you really do the conversion well. There were a number of turbo 2.2 done here originally, and I know of one. He had nothing but trouble, burning out clutches and continually having cooling problems, so the car could never be used for long. And I heard similar stories about the others. All this and the car was not as nice as a good 'S' in my opinion.
The performance envelope of a good 2.2 can take say 185bhp which is easily possible, and it will still be sweet. After that you are going to have to do major re-engineering.
Roy