Thanks! I just found the same tire on http://www.motorcyclemaniacstore.com/ and pulled the trigger right before I saw your message. I did try to use the modern sizing but everything I found was sold out, for front only. or tubeless. I thought I knew about tires but in the last 5 days I realized I don't know shit. Had to get new tires for 3 bikes(mb5. RX50, targa)! Shit gets expensive but better than riding on old cracked rubber. I wasn't sure about going tubeless and how that would work so I didn't explore that much.
If anyone else is interested here are a list of the tires I found that will fit and are not listed under SCOOTER TIRES. All of these had 3.5 x 16 Rear tires:
DURO HF319 CLASSIC VINTAGE (K70) MOTORCYCLE TIRES
Kenda K489 Motorcycle Tires
The heidenua looks the coolest to me but also cost $70 and is out of stock everywhere until mid to late august.