If you can find 2 lenses and make them into one it might be worth it, but when they have to start replacing elements they might not even be able to find the parts.
A few members have taken their own 50mm primes apart and repaired them themselves. From what I can tell most older Pentax primes are using a similar design.
I wouldn't tackle a Zoom lens.
Gentec can repair Sigma lenses, and theres a good chance they'll fix others as well.
They're the Sigma importers for Canada.
Here's their web site.
Gentec-International