The maximum engine speed is determined by the governor. The most common cause of low maximum speed is a stretched, missing or incorrect governor spring. I haven't seen the governor spring in your pictures - the only visible spring seems to be the anti-oscillation spring, which runs from the governor arm to the governor's throttle link, in your last picture. If you can post pictures of the serial and model numbers, and the cowl, we might be able to figure out what model it is and look in the manual. Otherwise you will need to look at the speed control linkage, where the speed control cable attaches to the engine, and post pictures of it.
