Have you taken the starter motor apart at any time?
I only ask as a lot of riders took KTM starters apart and then got the magnet polarity back to front and robbed the motor of most of its power. Very easy to do with that particular motor.
It could be the motor brushes that are either worn or sticking in their guides and not making good contact with the commutator. Worth a check but make sure that you mark up the casing for proper orientation on assembly.
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