If you are going to remove the tolex, use a good hot hair dryer or a heat gun and it will peal off prertty easy, but you cannot reuse it. You can buy new tolex and it is available in different colors if that matters. You need a little practice putting it on and you could use the old tolex as a templet. Also, you could remove the tolex, fix the crack and (if its a plywood twin cab) sand it down and stain it (no tolex).
In order to fix the crack/seperated seam, it's best to remove the tolex first anyway to be able to see exactly how it should be fixed...glue and staple, or glue and screws etc. If you cant use any fasteners like staples or screws, wood glue and most important clamp it while it drys.
You might want to think about purchasing an empty head cab for it and make or purchase a seperate speaker cab.