I have sailed the Windy14 and the Nacra 14sq a lot.
I think that following the H14 development and the Maricat style would be the most sensible thing to do.
Check the two attachaments...that Maricat sail isssswweeeeeet for for an older boat.
The Windy is a boat challenged by it's date of design and simply adding cloth might not help.
Way more mast rake, square top and mast head reacher would be a good bet.

Taking great care to lighten the rig would do the boat a lot of favours as it has very little boynacy and freeboard.I am clear that if you got the Nacra sail and mast that it would just overwelm the Windy.