Welcome to the forum !
I did a search on simplicity 12.5 lth, in parts diagram I don't see a shut off on fuel line, you can go to any parts store autozone etc, or hardware store and get a shut off valve, tell them what you are doing most will know size you need, or take fuel line with you, now would be a good time to change it, just cut fuel line from tank (it's better to put before filter so you can turn fuel off to change it) just remember it will leak fuel when you cut it ! I take a small pair of vice grips to crimp hose so it won't leak.
It doesn't take much pressure, don't crimp very hard or you will damage fuel line.