Can I change a 10hp generator motor with an automatic throttle to a manual throttle?

Yes, you will be able to control the throttle. Look for a lever that pulls on the governor spring. On some generators, the governor spring attaches to an arm with adjustment screw and locknut or metal tab that bends into position. All that you need do is to pull on the governor spring to increase rpm. The governor spring pulls on the governor arm that is attached to the fly weight governor inside the engine. The fly weight is turned by the cam gear in most cases and the force that it produces is constantly trying to close the throttle. The governor spring provides the counter force to open the throttle. Without the governor, the engine can over rev. Good luck with your project.