Refurbish or replace – mouldy helmet……

Having had my head protection for some 10+ years now, the foam is somewhat mouldy and nasty. I can get replacement foam from local retailers but want to know

A: Is it worth replacing it, and

B: How do I go about removing the existing foam without damaging the shell?

Can’t really afford to buy a whole new lid, especially as being the kit-tart I am it would have to be a custom graphic version!