Have you tried to take the clutch assembly off the engine yet? Are the threads standard or left hand threads? left hand thread would be better for keeping the adapter on , but just try and find an affordable left hand thread tap. [link=http://www.irwin.com/irwin/consumer/jhtml/detail.jhtml?prodId=IrwinProd100437]Irwin[/link] has them, no prices though.

Standard thread is acceptable as long as loc-tite is used to retain the adapter.