Harry already has a ready supply of compatible building material, the stones of the ruined lighthouse on Demonreach have a very similar warding, he can use those for repair AND to create a interior safe room for Maggie, two layers of warded stone.
It’s almost as though someone thought he might need it.....
The other repair job will be the cottage on Demonreach, make it a little less basic and a little more comfortable, plus use the rest of the Stone to expand it a little as a fall back position.
Whilst the Castle is perfect for standard training, the island would be better for combat training, or to stash other fugitives from the White Council, so fixing up the cottage also needs to be done.