sounds like you are getting power to the solenoid but the starter motor isnt getting a good connection of power to the communtator.
the solenoid pulls in and engages the bendex gear, thats the clunk you hear.
once you get the starter out try to get to the brushes and communtator. clean the comm with brake cleaner or electrical parts cleaner and a mild abrasive pad getting rid of any black carbon build up. make sure the brushes look good and the brush springs are solid.
apply power via battery, see if it spins.
you may be able to get it to start without removing it, try tapping the starter with a hammer to loosen the hold on the brushes. you can also remove the spark plugs, hit the start button to engage the bendex gear and CAREFULLY turn the engine over by hand. this will turn the starter over because the bendex gear is engaged. if it suddenly turns over you may have just had a sticky brush. but be careful not to get your hands caught in any moving parts.