I got it man
in case of CD70 the chain grade is same but u have to get new "chain gots" (consumable item) with new chain & some time "chain adjuster head" (adjuster takori) too so the main sproket/gear drived by the chain usualy remains same. But in worst case u have to change them too.
The cub engine for 50CC & 70CC are almost same with slightly deiffrent bore, valve size & gears. but in case of CD100 available in Indian & Pakistani market the over all engine size is bigger than those.