Re: 1991 Celebrity I/O 4/3 V6
Millions of those bellows have been replaced. Very common job. Go here and get the manual you need, follow the directions, should be #14, could possibly be #6:
http://forums.iboats.com/showthread.php?t=251571
No need to pull the motor, only the outdrive. Outdrive should be pulled every year anyway to check alignment, lube u-joints, etc.
Don't be surprised if leaking belows has ruined gimbal bearing, u-joints, input shaft, etc. Hopefully it hasn't taken out the upper seal on the outdrive and ruined it too. When's the last time outdrive oil was changed?
Easy job, but it does take a couple of specialized tools.