It appears to be a zinc or aluminium casting, so I wouldn't risk trying to bend it back to straight. My method for dealing with such things is just to use my floor-standing 14" disk sander, staying close to the center of the disk to keep the cutting speed low, using light pressure, letting the workpiece cool frequently, and moving it about on the disk so it won't clog the abrasive with soft ground-off particles. (You need a very coarse disk to make this work - I use 36 grit just about all the time, and 80 grit when I need to, I don't even own a disk finer than 80 grit). However before I had the sander I used a file, as Joe said. Because this flange is bent quite a lot, if you have to use a file it will take time and be very tedious unless you use an aluminium rasp, then finish with a file. I do not think you can achieve a satisfactory outcome with gaskets, it is bent quite a lot. Remember that you will get a lot of metallic dust in the carburetor: plug the hole in the center of the flange, and expect to have to clean the carburetor in detail afterward.
With regard to your idle speed, does the throttle butterfly close to the point where its external lever rests against the idle speed adjustment screw, when you set the speed control lever to minimum speed with the engine not running? If not, you have a problem with the curvature of the governor's wire link, or the length of the governor spring.
