That an old lawn tractor you got there.
First thing is check your transmission fluid level, when was the last time you changed it, and did you put on a new OEM filter?
Then check the pin that holds the pedal to the shaft (part #30 in yellow). See if the pedal and shaft are turning together or if the pedal moves and the shaft doesn't. If that's ok follow the linkage back to the transmission to see if anything else is loose. Check any springs and make sure they are not stretched out.
Note the grease fitting - part #75 - have you ever greased this? If not it may cause wear and tear on other parts.
