I would suggest that you reach out to members SS LS1 and John jr to see how they addressed the situation. If you look at both of their profiles, they have put together quite a few big bore ATC70's and would probably be able to answer your question
The ZB50, ATC70 and CT70 all use the same rear cush drive hub set up and the same rear sprockets. I have never heard of any problems and some have upgraded to 190cc motors. If your concerned I'd replace the rubber bushing in the hub. 20-30 year old rubber dries up over time. That's my 2 cents...
I run the cushion rubbers in my R55 ATC 70 with the Daytona 190. They are fine. They will wear out in time and abuse. Gordon Spangler now sells the sprocket hub to fit the ATC 70's that will let you run the CRF sprockets.