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It is possible to replace the exhaust bellows with the drive still attached but you will be in for fight trying to attach the new bellows to the bell housing (drive side). There is a small hole on the port side of the gimbal housing that will allow you to access the hose clamp at the transom side of the exhaust bellows (see picture below)...
Removing the drive is fairly simple (disconnect speedo connector, put drive in forward, disconnect trim cyliders, remove the 6 nuts at bell housing to pull the drive) but you will still likely need the expander tool (shown in the picture above) to install the exhaust bellows to the bell housing. You may want to consider installing an exhaust tube instead of an exhaust bellows (will not need to pull the drive if using an exhaust tube), the tube will only connect to the transom side and will have an open hole for the bell housing side (the only noticeable difference between the bellows and the tube may be a slight increase of exhaust noise as the bell housing side of the exhaust bellows is not secured by design).
Here is a picture of the exhaust tube (on the right) in comparison to the exhaust bellows (on the left)...
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