I would take measurements as best you can with the screen in place and then carefully break them out. It is likely that you won't end up with too many peices which can be glued together to form a template.
(Don't forget eye protection in case it splinters)
Breaking them is not a good idea and probably will not work, they are polycarbonate and very strong, not Perspex. I believe to remove you have to undone the screws holding the frame together and take apart. I would buy new runners as well. Good old Melvyn Rutter will have the sidesscrren Windows and runners. The polycarbonate can be ordered online on various thicknesses and cut with a special jig saw blade, but tricky to get a straight edge, can be finished off with a file.
