Heavy can be a drag In my opinion.
But it is also relative, do you mean heavy by windsurf standard or kitesurf standard?
A heavy windsurfer could be 10kg. That might be still be heavy after cutting down, if you have to add any reinforcement...
It would be heavy and a lot of work to convert an old windsurf board.
The back end of some sandwich construction slalom boards could work - especially if it had cut-outs underneath to reduce the tail area. It might work to attach the rails and underside of the nose to the middle and deck.
The simplest way would be to find one of the old kite race boards cheap - that had a deep tuttle box at the rear.
If the box is strong enough, the board can be used with a windfoil without modification.
Some of the Aguera production boards had powerboxes, but there is a powerbox head with a plate available for the Neil Pryde Glide windfoil fuselage with an XL front wing.
People have caught onto that and the boards are maybe harder to get now, but there may still be some to be found.