I agree, probably a pain unless you have one of the Stihl TS-50, 510, 760 types (1111 powerhead). The 50 I worked on had a pulley that slipped over the existing clutch/drum setup. Then you bolted on the cutoff arm to the bar studs. That was it. In a pinch I could have bolted that setup onto my 075. But that's alot of powerhead for a 12" wheel.
The drum/sprocket was brand spanking new, so I swapped with one of my used up ones off the 075. Figured he wouldn't miss it.
I know this info won't help you but it is neat to know.
Chris B.