That particular year, they did offer a Jag Special with dual carbs. Parts diagrams should be easy to find to look at the differences.
Things that come to mind that you would need include, throttle cable, carb, carb boot, manifold, oil line, etc. Because the Jag Special provided slightly higher HP and peak RPM, you may need to change clutching to take advantage of the different in revs and hp.
My personal opinion is that the sled you have there is almost mint. I would leave it as is and run it. To add a second carb will only slightly increase hp, and increase fuel use. That engine is bulletproof. Run it and enjoy it!
I would just sand blast the skis, paint them, and run the sled.
THE MODERATOR DIAL! Keeping Spammers in check!!!!!