Hi - I can't answer your S2000 questions, but as it pertains to your question pertaining to the S+ chip sets and if they came in 100 or 120ns.
Given that the S+ was last made in the late 80's (a few in the early 90's) most are 40yrs + now and the original chips were not that fast.
SO.... The easiest answer is to burn the desired binary onto the chip size/speed of your choice.
You can get an original game kit - this means your legally buying the game and not copying or infringing on anyones copyright.
A game kit is Glass/Strips and Eproms.
You read the original chip into a binary file using a Eprom reader/writer, then you write it back out to a faster chip.
To that point 27C64 chips are almost nonexistant and tend to be on the slower side.
Even the 27c256 chips are pretty much obsolete - in that they are not being made anymore.
The good news is that there is lots of NOS (New old stock) around and you can also use 27c512 (just a larger chip).
https://www.mouser.com/c/?q=AT27C256R-70PUhttps://www.mouser.com/c/semiconductors/memory-ics/eprom/?q=AT27C512Most are in the 70ns speed (The lower the number the faster the chip) and no harm in being faster.
To use a larger chip what people do is take the 64K binary, and write it 4 times consecutively to the 256k chip (or 8x with the 512k chip).
When your writing the burner writes from the bottom up, while the machine will read from the top down. So your choices are to write with an Offset so the data is higher up in the chip or to simply stack it (write the same code 4x times) and not worry about complicated offsets.
If your not up to this a reputable member like SixCardMark cand probably provide you the eproms at the right speed.
Just thought you would like the info.