I’ve done this kind of stuff before. It was a recording of a classical singer with synphonic orchestra produced on 78rpm vinyl. There was a lot of background noise and crackling.
Following the recording in Cubase made in 24 bit, 44.1 kHz, I needed to eliminate crackling and background noise as well as possible with the tools allowing to carry out this kind of task and without altering the recording too much. . (Currently Waves has a suite of plugins dedicated to this) The process that followed was the equivalent of mastering.
The first step was to restore proper equalization in order to improve the overall sound of the recording, as the process of removing noise in some way alters the audio quality of the recording. Lots of patience and time.
The next step was to restore the dynamics with good compressors and dynamic EQ in certain places.
In this case, due to the fact that at the time the recording was done in one room and in mono, I needed to improve the sound image in order to give it some space.
Subsequently, I worked to control the overall harshness of the recording by eliminating unpleasant high frequencies. Next, I added a slight saturation to give a little warmth and finally used a limiter to raise the signal to a suitable level for the musical genre.
This had given a very correct result that I was able to burn onto CD afterwards. It was a gift for the person who sang on this recording who had reached a respectable age at the time.