ul need to take the left hand engine casing off, drain the oil over night,remove sprocket and gear lever then just undo all the bolts (there are a few) and put them down in order they came out (there all diferant sizes) then undo the starter motor bolts and cable, disconnect the cable comming from the engine to the main loom.
you might need to get a new gasket but if your carefull and can get hold of some gasket in a tube (like glue but made for gaskets) then you might get away with it.
gently prize the case open it will go so far then feel like its stuck, as long as your sure there are no more bolts holding it together then dont worry its just the force of the magnets in the fly wheel.
once open you will see the starter gear to the top left (if it hasnt fallen out) and the flywheel in the middle.
now its time to tap the starter motor out from the inside (its a tight fit!) but make sure you take the bolts out.
to put back together put the starter motor in 1st then the gear then put the liquid gasket on the engine casing and fit, be carefull of where the cable comes out of the casing from the stator.
put all the bolts in and oil, connect everything up then start it and let it run for 10 min, look for any oil leeks! if you get a leek then you will need to take everything appart and put some more liquid gasket in that area.
it would take about a day to strip it and a day to put back together then how ever long it takes you to dismantle the starter motor and fiddle.