If it has a Sears door opener and those gears are trashed, you can buy a set of gears for less then 10 bucks or an entire rebuild kit for about $25 - and ebay is the least expensive, whichever way you go. It will take you about 2 hours the first time, 30 minutes the next.
As far as a new spring, get both sides. Both HD and Lowe's carry them. Take one of the old ones with you to match.
Safety cables saved me from a clonk on my thinly haired scalp - again, HD or Lowes have kits or cheaper yet, some cable and clamps.
I used the full gear kit, but also have rebuilt this shaft and gear set for the next time or for the other door: