As long as your o-rings seal properly I don't think it really matters what you use. I don't think there's any type of standardization to these builds, just functionality.
I am not sure but I think fender washers are flat washers but a little thicker and with a larger hole in the center. Whatever will take up needed space to make everything tight and leak proof.
Great, thank you guys, gonna jump on the snowmobile and head up to grab some of those kits with the flat washers in them as no one carries the fender's around here.
I'm also going to get a different o-ring. I'm not please with the quality of the one I got at home depot originally.