Renault ares poor brakes

[Ren ares 620rz]

Looking for some advice on converting brakes on 620 rz 1999. Brakes are shocking and been told that the later models have boosted brakes. Is it a option to covert the master cylinders to the boosted brakes without changing any internal parts.
Any advice appreciated

 

[marshfarmer]

Brakes are ok on mine but better on the 640 as it's a different system. Do they need bleeding?

 

[Wellytrack]

Could be worth checking out. Brakes should be more than adequate for its era.