The missing instructions can be resolved at compile time using a modified build of GCC or by generating traps at runtime on encountering the unsupported instructions.
You can check the source code, FPGA project and FPGA bit files at OpenCores.
And here is an example of an MP3/WAV decoder that was made with this MIPS-like processor, the project is an FPGA based MP3/WAV Player using just a FPGA, some RAM & a stereo DAC.
“The project consists of a custom 32-bit soft core processor running at just under 60MHz which decodes the MP3 algorithm in software with no hardware acceleration apart from a single cycle Xilinx multiplier unit.”
The original article is complete with a full hardware and software specifications, source code, schematics and much more…