Hi,
Had same issue with my 85 F6, its the 18 tooth cog if your gearbox hasn’t been changed from standard, (can appear green/blue in pics), had to search far and wide for a new one!, finally found one stocked in Italy, from DKS Automotive,Renault and Citroen specialist.(NOS Renault part)
Do belive upon further searching they were available from a 4 specialist parts place in Holland, but cant remember their name now.