Merc has a kit to convert to the electric tiller - Merc part 94185A18, available through any dealer - just be sitting when they tell you the list price is $271.88.
You may want to try Ebay or a recycler. The tiller handle from the post 1986 6, 8, 9.9, 15 and 20 horse models will work for you but I believe that only the 20 and 25 came with the electric tiller (and maybe the "super 15"). If you can get one complete for 50 bucks or so you can pick up a starter switch as an add-on.
I have a handle from a Merc/Mariner (so it's grey) that needs new cables and is missing some parts on the linkage end of things which means it could cost upwards of 150 bucks (the cables alone are 40 bucks and their are a number of parts that "don't get removed when converting from tiller to remote - which is where this came from", but if you can't find anything suitable and are interested we can come to terms (I will let it go cheap or trade for your left-overs after you convert), but you would have to buy/scrounge possibly the gear and the linkage (not sure exactly what I have)...