Well, no need to "emulate" Symbian OS if the source code is available, it would be easier to just port it.
However, 2 concerns (maybe unfounded):
1 - Wikipedia "Nokia released Symbian under a new license and converted to a proprietary shared-source model as opposed to an open source project", so I don't think ALL the code is readily available for a port (I might be wrong though).
2 - Symbian currently only supports x86 and ARM platforms, so it could be quite a lot of work to support MIPS. I might be wrong again.