Great, thank you!
Can I use faster than original (55ns instead of 70ns)?
It should be fine when it comes to data access.
Note that there are other parameters that have timing changes. For instance, t
CHG and t
WHZ. However, these shouldn't be a problem unless it is being accessed by something outside of a typical processor given the (for these days) relatively slow access.
Also compare the output load rating to the original part. This one is listed as "C
L = 50pF + 1TTL". For instance, if the slower original is rated to 100pF for it's timing, etc. then it may all balance out anyway.