Doc Sprocket
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To convert your starter you would need a new crank shaft that is made for a cup.
Actually, if you were careful and calculating about it, I think you could just cut the extension off the shaft just past the threads and use a standard nut to retain the new cup. (I think it's 1/2NC20)
You'd have to do some careful engineering to center up and square the new recoil unit onto the old blower housing, but I think it's feasible.
My Tec H60 recoils use a pin that centers the recoil in relation to your crankshaft. Using one of those would help greatly.