It's actually pretty easy to rewind a pullstart; I used to do that type of repair all the time.
If the spring isn't in the housing, re-insert it into the housing. The hardest part is when it coils into a ball. The easiest way to fix this little mess is to throw it gently against something soft (a pillow works great here). It should hit, bounce off, and literally un-coil back into a spiral. When it works, it's the coolest thing you'll ever see.
Once it's back in spiral form, wind the spiral a little tighter, so that it will fit into the case; the goal here is to get the spring into the case; just make it fit at this point, and in the right direction. It should spiral/tighten to the left. Ensure that the spring is engaged into the notch in the case.
The hardest part is re-tying the string to the spool. Thread the string through the opening in the pullstart housing, and tie it to the spool (you may have to re-drill a tiny hole in the spool).
Once you get it nice and firmly tied (use as small a knot as possible, and put a small drop of CA on it to secure it, allowing to dry), re-wrap the string in the proper direction.
This is determined by holding the spool between your middle finger and thumb. Place the part that faces the engine (usually has a hex or a square in it) towards your middle finger. Now, turn the spool to your left. At this point, neatness doesn't count; just get the string wrapped onto the spool as much as possible.
Now, here is the real fun part; insert the spool into the housing carefully. Make sure it engages the spring. Next comes the tensioning of the spring, which actually isn't all that hard. Simply pull the pullstart handle enough to rotate the spool once, and while holding the spool, use a small thin tool (a dental pick works awesome here, but a toothpick or straight pin will also do) to pull the string from between the spool and exit hole. Now, re-tuck the string around the spool. Repeat as much as needed to get the proper tension (might take two or three times).
Secure with a rubber band or tape until ready to reinstall.
It's a lot easier than it sounds to rebuild a pullstart. Just take it slowly, and you'll get it.
Now, once you get it re-wrapped, practice the proper method of pulling on the starter; just short little tugs, not full arm extensions. Flicks of the wrist work the best, just enough to pop the motor over 2 or 3 times.
