I have never had any faith in the wipers on the S2 or Coupe.

They park on the bodywork, taking out the wiper rubber and the windscreen rubber in some cases. I have to fit new ones every year for the MoT if they have been used in anger.
The variation in wipe pattern is much as you describe - uneven and mainly on the edges.

I have looked for any form of replacement but not been able to find anything. I even looked at hack down some of the 'blade' style wipers but that started to damage the screen surround.

Everyone loves a Morgan. Even me, unless it's broken again.