There is no conversion kit. The basic pieces you will need are: extension case, associated studs/nuts, new longer drive shaft, new longer shift shaft, and new longer water tube.
You will have to partially disassemble the lower unit to change the drive shafts. You will also have to shim it properly.
You will have to partially dismantle the midsection of the engine to change out the water tube.
I recommend changing the water pump at the same time if you have not done it for a while.
Parts are around $800 - $900 plus about 5 hours of labor or so depending on how difficult it is to get apart.
Mike