Should i dismantle the whole chimney crown?

Rupali, you're at a juncture. It's either replace it all or a carry out a repair clearly - but I'm not sure of the gain from a complete replacement given that chimneys are less commonly used these days anyway. But that depends on your outlook, intent and availability of time investment, etc.

I'd say that if the capping and pots are stable on the left then I'd just fashion a repair using old bricks and then cement over it to make up the form of the original capping. I'd do this using cut-down socketed flue liners or 'footballs' (half breeze-blocks) and whatever bricks you have and a good mortar mix. Don't let the throat narrow down beyond what it's already at, obviously. I'd bung up the chimney with an old coal bag, and make sure that nobody lights a fire, etc. I'd also work in strips of glass mesh to the capping to strengthen it across the width, especially around those cracks. I'd rake them out well too and wash out and then backfill them with a cement slurry and then a mortar. They need to be washed out due to the loose and acidic compounds and dust present.

https://mybuildingsupplies.ie/monster-mesh-glass-fibre-reinforcement

I'd avoid working with mortar at all if there is any chance of temps below 3-5C (as the chimney will be more exposed). All mortars should have a plasticiser added and I'd avoid using the old bricks as they will be full of compounds. Waterproof the cap then once it's set.