Hello Richard, only just read this post. I might be wrong but is the 4hp Mercury the same engine as the Mariner and Yamaha. There are often Mariner engines on ebay, often dismantled too, and lots of old Yamaha engines. If so you should be able t get hold of a knackered one. It seems likely that the shaft would outlive the powerhead. The spare parts seem to be extortionate considering that this is one of the most mass produced engines of all time and is widely used all over the world. Paul