2003 dt125r electric start issue
Make sure the starter motor is fastened appropriately to the engine.
Make sure the starter bendix has not come away from its housing.
Ensure the teeth on the flywheel are secured.
2003 DTR's did not have starter motors. It was on the final revisions of the DTRE's that had the starter motors.
So it sounds like you've had an engine/loom swap at some point?