You should get a stone arch or an angle iron to cross windows or you can also use a stone lintel if you can find one. But it can be very hard to build these windows by yourself, so I think your should hire a window repairman.
:2cents:As your words you can repair it with any carpenter or if they are out of order, then I think you should go for vinyl or aluminum windows. They are much better and efficient as compare than these wooden windows.