If your goal is to install a speaker that replicates the original, then you should look for an old / re-coned Eminence or CTS. These are the speakers that Sunn used back in the 60's-70's. But, this may not be what you really want.....depends upon your desired use, playing style, type of music you play etc. Sunn also offered speaker upgrades to JBL D140F and Cerwin Vega for some amps, but perhaps not for your specific amp.
So....take all of this with a grain-of-salt......if you are looking for the original sound, then listen to the old guys like EdBass, LoudThud and Soundmasterg. But, you know that it's only a speaker. If you don't like the sound, replace it! Most of the fun of this hobby is the quest for tone! FYI...I own an early 70's Stage Bass with the original re-coned Eminence....sounds great! I also own a 200S and 2000S with matching JBL D140F cabinets. Different animals, but they all sound great.