Easy way to check is to strip the machine down and remove the clutch, then turn it on.
Does the motor work ok? If so then it is the clutch.
However in my experience clutches NEVER jam up so bad that they slow the motor, and if it did the motor would chew through the belt very quickly.
New motor will sort it, and as russ said, a new filter too, and wash the washable one to be sure! When you take the filter out that sits ontop of the motor housing is it covered in black flth underneath? If so then the motor has chewed its carbon brushes up and is