I suppose in principle it can be done. You'll need a new mid-section and prop drive-shaft for starters. You'll need to access the gearbox to release the prop drive-shaft which means taking off the impeller, waterpump, oil seals, to access the pinion nut on the drive-shaft.
Would be a lot easier if you could source a complete gearbox with drive-shaft in place. After reading this - you'll see what I mean.
Apparently you also need a Mercury special tool 311875 drive shaft socket to get your pinion off without damaging the drive shaft with the vice. And you really need it to re-torque your pinion nut.
More details here. http://answers.yahoo.com/question/in...2071029AAk9AJj