[Stilkikn]

Appears that most of GZ gurus have moved on. I've only had mine for a few weeks and have not changed any seals on the GZ so can only offer my opinion for whatever that's worth. First things first, are you sure that the oil you're seeing isn't oil dripping off the chain? Pretty important to positively identify oil leaks before diving in.

It looks to me like all you'd have to do to change that seal is remove your shift lever and maybe the linkage bar and then the seal would be easily accessed and changed. As far as removing the engine sprocket cover I don't think that would be necessary but might give you a little more working space. But it's only three bolts to remove it and only takes a minute or two so why not remove it and that way take a look at the condition of your sprocket at the same time. Apply some grease to the lips of the seal before installing.