Parscavenger is a program to aid in the recovery of par2-protected data. It doesn't do any super-math with the actual parity information. It just uses the hash values to tidy up what data it can. Feed it with a par2 file from which to get hashes, and whatever garbage data you can hope to recover something from - corrupt files, disk images, incomplete downloads, even not-quite-the-same files like earlier versions. Out of it will come, if not a complete set of data, at least a set of files with the correct name and with the available data correctly placed within them. Suitable input for other utilities.
Just compile the source and do as it says.