There's also
Elmer's Auto and Toy museum in Fountain City, WI. Super-nice family owned private collection with lots of unique pieces and one of the most beautiful roads in the nation taking you there.
If you're headed to Sturgis, there's a great Auto museum in Murdo, SD on the way - used to have lots of vintage bikes. And there's the Black Hills Motorcycle Museum once you get there.
And if you're open to something completely different, there's
Dr. Evermore's Forevertron in Baraboo, WI. It's not a motorcycle museum...but there is nothing else like it.