I installed a band brake on a clutch today. I didn't have a mounting bracket. It was an older 3hp Briggs motor. I removed the second bolt from the motor housing and used a longer bolt with a nut on it to hold the brake in place. I actually used three nuts -- one against the crank case, and one on either side of the band brake. It worked really well.
I don't know if this method would work for other engines, but for small briggs, this is a really effective and easy way to install it.