Does the engine start if you push and quickly release the kill button when the starter is spinning?
This is something you can do on the KTM 2 strokes with external starter. This tricks the CDI into producing a bigger spark, if I remember correctly.
The KTM starters would progressively slow down due to friction build up in the bendix and crown wheel. It might be worth stripping, cleaning and lubing the drive train to eliminate that possibility.