Am I the only laughing at the idea of a "kit" to unconvert a bike from a homemade single speed?
Put a rear derailleur on that hanger, a cassette on that rear wheel, and a new/longer chain on. A downtube shifter to match the rear derailleur with a cable and some housing will make it work. It will be more than 3 or 5 speeds, but I think you can deal with that. What cassette is your rear hub designed to handle?