I wouldn't run the engine without the exhaust pipe, that gas coming out of the hole is around 1000F and nasty if it gets you or a drip from a leak in the fuel valve should pass through it. Find a good hard metal surface and use a hammer to make sure both ears on the flange are flat. If they are in line and there's a bit of weld sticking out where the flange joins the pipe a bit of work with a 10" mill bastard file will flatten it. BTW no joke, they are called a bastard file, a 10 inch one is about an inch and half wide with single cut teeth. Single cut means the teeth run diagonally across the file double cut are criss cross cut and don't leave as smooth a finish. You won't embarrass a hardware man to ask for one in a good hardware store.
Goldwing 1500, Silverwing 600, Schwinn 49