Stihl parts certainly aren't cheap, so it may be cost prohibitive to rebuild them with new Stihl parts. I'd keep an eye on eBay looking either for those parts or for a HL-KM 135 selling "for parts or rebuild". Look long enough and you'll find one. I've got a Husky 326 Articulating hedge trimmer (goes for something like $450 new) I got for less than $150 by buying the pole/hedgetrimmer head first w/o the powerhead, then keeping my eye out for the powerhead.
You can often buy a whole non-working piece of equipment for less than the price of the parts.