Replace the original with a new one Coolant Expansion Tank, MX5 Mk3/3.5/3.75 – MX5 Parts
DON’T bother with a fancy metal one at a far greater price.
What’s the mileage? Have spark plugs been replaced, coolant etc.All depends on age/mileage.
Loads of threads on internet re rust Buy MK3 - How much do they rust? - #18 by Sunzoffski