"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.
I converted my short shaft etec 115 to a long shaft (the dealer sent me the wrong engine and then would not send me the correct engine) by installing an extension kit from Bay Mfg which the dealer provided for me. The kit I used was model 506. It worked fine. It does require you to modify your engine slightly in that you have to drill out the space where the drive shaft passes through the mid section so that the wide end of the drive shaft extension piece will fit through. You use a 1" metal holesaw on a long extension to do this. It takes about 45 minutes of careful drilling. The whole installation can be done in about 1/2 day I would guess.
Then you just assemble it all.