Does the engine actually spin at normal starting speed when you try and start it ?
If not, it's either the starter motor, solenoid, or one of the connections.
If it spins OK, then whip a spark plug out and see if you've got a spark. If you haven't then it's within the ignition system.
Check you are getting fuel to the carb.
Check the cam belt hasn't snapped.
Try bump starting it.