If you haven't yet bought the brush send me a PM with what size 17" 18" 19" hawk machine you have. I have some shampoo brushes that are new in my shop. I think what I have are 17", but I don't remember. Also I have a box full of clutch plates that fit the advance low speed.
I know powerflite has universal clutch plates too. There is actually 4 or 5 different ones. I think the only problem I've ever encountered is when the clutch plate was a different diameter than the riser on the pad driver. I think most pad drivers have 5" holes to mount your clutch plate too, but a tape measure will give you that. Never let a silly thing like fitting a pad driver to a buffer slow you down.

I have a couple of Blue Star scrubbers (I think hawke made some of those). I never have done anything but HWE