YSZ-4 clock firmware similar to original. Feel free to use it.
--- Compile using MCU 8051 IDE. ---
By the way, AT89C2051 doesn't require any fuse bits. It has only LOCK bits.
I have no experience with AT89S2051. My programmer doesn't support it. If you have any suggestions, write me to e-mail pointed below.
Good luck!
aleklom(at)gmail(dot)com