Mario,
I am sorry answering late.
IMHO there are no uprated clutch springs, except perhaps some custom made ones. Did your clutch slip? Normally, the clutch works well when within the factory specifications and of course no contamination oil/grease. You'll find a very good article about the clutch from the late Morris Parry in the Augusta Newsletter Nr. 16, October 2013. In there, you will find how to modify the driven plate by using a part from a Ford Escort Mk1/2. I did this with great success only to find out that a new driven plate can be bought with Cavalitto.
But more important you will find the dimensions for the cutch parts and possibilities to increase the pressure. I have done a slight modification by shortening the 3 studs (31-1010) by 3 mm.
For my "spare" engine I bought a driven plate from Cavalitto. It works very well.
Let me know, if you need a copy of the Newsletter.
Karl