If you break your pull start we would recommend you purchase a new one as fixing them can be quite difficult. You can get a new pull start here or you could go for an electric starter instead which are available here.
If you do want to attempt to repair it you can follow these instructions:
Remove the recoil starter by removing the four screws that attach the started to the engine. Next remove the one-way hub from the center of the spool. Untie the knot that holds the handle on the cord. Let the cord pull into the housing. Next Carefully disassemble the recoil using extra caution with the recoil spring.
To reassemble the starter, start by inserting the end of the spring into the slot on the starter spool. Slowly wrap the spring around the inside of the spool like the picture.
Insert the spool into the recoil housing. Insert the end of the cord through the hole in the outer casing. At this time tie a temporary knot in the end of cord.
With the cord through the hole and the spool inside the housing, its time to coil up the spring. Using a pair of needle nosed pliers, grip the spring through the center of the spool at the bend. Make sure the cord is not pulled into the house or you need to start all over. While holding the center of the spring turn and coil the spring clock-wise to tighten the tension. Turn the spring one full rotation and put the end of the spring into the spring mount in the center of the housing.
Now you need to put your handle back onto the cord. While holding the cord from being pulled back into untie the temporary knot that was tied in step 2, Slid the handle spacer then the handle onto the cord. You now need to tie a double knot into the end to prevent the cord from being pulled back in. Please note it may be difficult to hold the cord and tie a knot at the same time. You may want someone to help you. Your recoil should now work correctly.