So in the beginning, back in 1972, the AMA had 2 classes, 250 and 500. In 1974 they added the 125 class, which gave us 3 classes. In 1994 they discontinued the 500 class and we were back to 2 classes, 125 and 250. In 1997 they increased the allowable displacement capacity for four stroke engines, so we ended up having 250 4t's racing 125 2t's and 450 4t's racing 250 2t's. Then in 2006 and 2009 they renamed the 2 remaining classes and we end up with a 250 and a 450 class. So, all the AMA has to do is increase the allowable displacement capacity for two stroke engines much like they did in 1997 and add a 125 class like they did in 1974 and we'd all be happy.
Oh yeah, from what I learned the AMA doesn't even own the AMA Motocross Series. In 2008, the AMA Pro Racing series was sold to the Daytona Motorsports Group.