If you could analyze the old ga.js and figure out a way to accurately duplicate the way the old utmz cookie was created, you could probably make some money off your effort. I've yet to see a great solution to this.
How old is "old", and what do you need it for? I've written code to parse the utmz cookie and separate code to recreate the whole organic/referral/utm attribution model in GA using first party log data. The only major drawback AFAIK is that adwords utm parameters are obfuscated and Google doesn't provide a way to resolve glclids to their campaign's utm params.