You don't need a special tool, just something to stop the sprocket turning when you undo or do up the bolt. You can wedge a screwdriver through a hole.
You need to release the bearing cap, which means slackening all the others too progressively to avoid risk of straining or breaking the camshaft.
If there is wear ridge or scour mark on the shaft where the seal has been running it's a good idea to put the new one in a slightly different position. I believe I have read a tip somewhere to simply push the seal right in and put another on.
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