Hello,
 This question will show my ignorance, but I'm still curious.  I have several old torrents stuck at 90+ percent.  Would it be possible for someone to write a program that throws random bytes at those torrents and let Tixati see which random bytes work and which ones don't work?  Thanks for any replies.

Let's say you have a 1 MB block of unknown data. That's 8388608 bits that can have 2 to the power of 8388608 combinations total. Let's say you have a top tier GPU that can calculate up to 100 Gigahashes per second, or roughly 2 to the power of 26 combinations. To check all possible blocks, you will have to run the calculations for 2 to the power of (8388608/26) seconds, which is still longer than any sensible timeline inside our universe.

What's worse, a single 160 bit hash corresponds to many different blocks. Roughly 8388608/160 in our case. Unless the block is relatively short, and has some known data in it (text or binary markup expected at certain places, for example), you don't know which one was the original.

What you can do is to look at files and their formats. Text comments and advertisement links might not be important. Video files can sometimes be re-indexed or partially restored with missing parts cut. However, if critical data is missing, you probably can't guess it.

You can also search for file names in BTDigg and other engines to check if similar torrents with same files are available.