70cc bike will only give good average if your speed will not increase 50km/h by riding single person.
Original air filter need to be used.
Personal experience was never good with cd 70. Its actually a fake idea regarding fuel efficency.
These are my views any maybe differ from others.
I drive it through Lahore to china border, Skardu, Deosai and Lahore (solo trip). But during trip I never concentrate on fuel economy the only thing I want was performance. As per engine its performance is good, but again, in performance mod there is no fuel efficiency.
So decide what you actually need, performance or fuel economy and then proceed accordingly.