clutch - the lurch could be evidence of the clutch going - or the need for clutch cable adjusment (at the lever - there should be about the width of a Nickel free play before the lever 'engages')...as for the replacement - it depends on what the basket looks like - you need to drain the oil, pull the cover and inspect. Its not too difficult but if you've never done it, take it to a shop or find a rider who knows it.
oil seal - there is a seal that you can get to - usually there's an oil seal, a collar and an o-ring - you should replace the set - again not overly difficult if you've done it before but you can do some damage if done wrong.