I've uploaded some 68302 assembly language source I did years ago for hacking the Practical Peripherals' ProClass modems. A book reader emailed me that he had several of these and pointed out that I had not included the source for the initialization.
If you can still find one of these modems, you can convert it to an excellent 68302 development board in just a couple of evenings. Full details are in my book, Build Your Own Robot!; look for the chapters on hacking the PP Pro-Class modem.
Download checkout.asm, the test code for the modem hack; this is a ZIPped file.