Re: New here.
I wouldn't think the gasket is any big deal It is really there to grip the spokes to keep it from sliding around I think. You will be mashing it flat with the back plate anyway. Mostly you want the slit in the gasket to be center of one of the back plates.
Frame adapter thing, if you look around on the site there are several ideas for them. One is to use a muffler clamp to make it work. That one is my personal favorite. You would need a couple of bolts from the hardware store to use in place of the studs in the engine mounts, and of course the muffler clamp. But you could do it with an electric drill and not have to drill the frame.
My posts have entertainment value only. A bike ain't yours till it has your blood on it. Then it owns you.