Rock-Ola typically used 16 ohm tweeters in an 8 ohm setting to give a cheap form of padding of the highs.
The originals were Atlas (IIRC) and very recessive although washing them did often help bring a little brightness. This was pretty much the same horn that AMI and R-O used on multiple models. When speakers are scavengered it is usually only the 12" that are taken, so there should be a lot of them around.
If the original is missing I suggest going with a modern horn such as this:
http://www.pyleaudio.com/sku/PH44/4-x-4-Horn-Tweeter. Since it is smaller in width you can add a filler plate, or use two in series.
The real improvement in sound requires replacing the crappy ceramic cart with a magnetic and preamp then removing the crude torsion spring used to "counterweight" the tonearm and adding a real counterweight. This will not only sound -much- better but be a lot easier on your records.
I had one of those little machines and while I left the original tweeter in-place I added a new square horn along with the arm mods and a 6 db crossover to get the highs out of the woofer. This effort took it from sounding like a cheap compact juke to a nice little machine which was genuinely pleasant to listen to.
Rob/NYC