I acquired recently a Navitimer 2100 Pluton and a 2400 Cosmos GMT both of which I was aware had issues with their ESA 900.321 movements. The long accepted advice has been that the movements are obsolete and that cannibalising old pieces was the only replacement option. Well perhaps not. A guy called Neil Rawson, originally from the UK but now in Sydney Australia has developed a replacement module (primarily for Heuer pieces but also for the 2100, 2300 & 2400) which is a like for like replacement that works. He is or has also been able to sort out issues with the watches i.e. replacing spacers and generally problem solving getting these 40 years old quartz dinosaurs operational. The guy is very good and very knowledgeable.
The cost is AUS $450 plus shipping but he has breathe life into these watches of mine (even repairing the small UTC module) He is definitely worth considering.
Regards
Bob
https://www.rawsar.com/esa-900-231-h4-module.html