Hi,
the 8051 and the 80251 are two different animals. I would not know that Atmel ever did a 251. The 251 is a superset of the 51 with 16-bit mode.
Now to your question about 51-simulator.
This is part of the Keil microVision (uVision). Even the version you can download for free has a working simulator.
Hope this is helpful, Bob