5th gear is a super simple swap, but you have to remove the transmission to do it. If you have the engine torn down, no biggie. Make sure you measure the shaft diameter though, as Mike has 2 different sizes and you might think you have one, but in reality need the other (ask me how I know!).
Installation is a matter of removing some snap rings, sliding old gears off and then back on in reverse. But it's certainly not something you can do in an afternoon (unless you're just REALLY good w/ these engines).