You can usually just change the head, check the model. A couple of screws hold it on.
When that is removed there will be a shaft stuck out, a bit of turning with pliers may loosen it off.
But a seized valve is what knackers the motor. There is a rubber ball that sticks to the inside when not used for a while.
Generally it is the heating valve as it is left off all summer. Good idea to turn it on now and again during summer.