The 1901 can take both RGB and Chroma/luma video as input.
The C128(D) uses the RGB for 80-columns display and Chroma/luma for 40-cols...
So what you need is a cable that you attach to the C128's "normal" videoport (the round DIN one) which should (in the other end) have one RCA-plug for Chroma, one for Luma and one for audio.
Once you got this cable you can use the RGB/Analog (80/40) switch on the monitor
to choose which video-signal to use.