Sounds like the starter clutch isn't disengaging. They're usually located behind the alternator rotor (flywheel).
To get at it you'll need to drain the engine oil (or lean the bike onto its right hand side).
Remove the left-hand engine cover (probably after removing the sprocket cover). The alternator magnets are quite strong and tend to hold the cover on so it can be quite tough to remove. Tapping around the casing with a rubber mallet should free the gasket joint, but you'll need a good firm grip to pull the casing off. Don't use anything to prise it off as you'll damage the oil-tight joint face.
To remove the alternator rotor you'll need to buy or borrow the correct tool - or make one yourself. U legged puller don't work on these and can damage the rotor. You must not hit the rotor as this can demagnetise it and do weird (bad) things to your charging system.
I *think* the starter clutch is on a spur gear that should be visible once the alternator rotor is removed, but can't be certain as I've not worked on that engine.
arrrf.
