I found online the disassembly of the ZX Spectrum ROM, probably derived/copied from the release by Dr. Ian Logan and Dr. Frank O'Hara in 1983.
Since at the time I was developing ZX Spectrum code, I became interested and after very few changes, the source code assembled successfully.
I am providing here a package which will assemble the included source code out of the box. Use it to make your own custom ZX Spectrum ROM - even if the modifications are as simple as changing the copyright message!
Downloads
ZX Spectrum ROM source code and assembler - unzip anywhere and run make.bat.