If the car is a good enough deal you could always convert it back to a solid roof.
Find a junkyard Mustang from the correct years and cut a larger panel out of the roof. Take it to a first rate repair body shop and have the sunroof removed. Trim the patch panel to match and tack it in. Then stitch the rest of the seam. For the inside you will probably have to buy a new headliner. When you are cutting out the panel from the junkyard car check if there are any reinforcement bars that were in the area where the sunroof was installed. Cut those out too. Make sure they are a lot longer than what you need. There may not be a l;ot of choice on how they get welded back in. After all, they keep your head off of the ground if you flip the car.
When all the welding is completed it's just bodywork and paint.
Find a junkyard Mustang from the correct years and cut a larger panel out of the roof. Take it to a first rate repair body shop and have the sunroof removed. Trim the patch panel to match and tack it in. Then stitch the rest of the seam. For the inside you will probably have to buy a new headliner. When you are cutting out the panel from the junkyard car check if there are any reinforcement bars that were in the area where the sunroof was installed. Cut those out too. Make sure they are a lot longer than what you need. There may not be a l;ot of choice on how they get welded back in. After all, they keep your head off of the ground if you flip the car.
When all the welding is completed it's just bodywork and paint.