Is there any (known) way to upgrade the old ECU of a MK1 Fabia? I'm guessing it has a pretty old hardware configuration and it might be not too responsive according to the current technology. Combined with a propper remap, it might have pretty nice output IMHO. 

What engine do you have to start with?

It's a 1.4, 50kW.

You can fit any aftermarket ecu you want,but you may also need to add extra sensors/upgrade yours to get it mapped properly/better.

This sounds nice. An aftermarket ECU will be able to speak with the other modules, such as the comfort unit or the other ones ? Where can I find one ? I have a local shop where I can get the best map. 

I don't think you'll find many that will talk to the rest of the car. A lot tend to piggy back aftermarket of the original.

Unless your going mad cams, throttle bodies or charged then I don't really see the point.