plastering over mortar repairs

[Madfish]

Hi all, I am helping renovate a 100 yr old home for my son. We have got large cracks where we have repaired stonework to considerable depth, digging out old mortar and remortaring, sometimes through to the next room. The wall is made of stones and covered with a hard plaster. So what is the best product and process to now cover these mortar repairs ,some which are over 30cm wide.