MTD trimmers are made by Ryobi, and most repair shops won't work on them or stock repair parts due to the high cost of repair versus the low cost of replacement, so make sure you have some source of supply before attempting to repair!
To reach the starter assembly, you would need to remove the trimmer shaft, romove the throttle wire from the carburator,remove the screws holding the plastic housing to the engine, then there is one T-20 Torx screw holding the clutch drum onto the clutch. then you need a special tool or an impact wrench to remove the clutch. Carefull! It has left hand threads! Then remove the screw holding the starter pulley on & slowly remove the pulley as there is a rewind spring under it. You should ALWAYS replace the spring. You won't want to do this twice!
The new spring comes pre-coiled. Note carefully how the old spring was wound and using needlenose pliers, install the new spring.
Then, using a NEW ROPE, wind it around the pulley in the direction of pull and allow three extra inches to allow for the handle and knot. Insert the pulley onto the spring, making sure the center of the spring fits into the notch of the pulley. Then, wind the rope once in the direction of pull and thread it through the rope grommet in the housing. Now install the handle and reassemble.