A MultiChain-formatted private key is just a way of representing the ECDSA private key in text form, and it's derived from it in any cryptographic sense. So I'm not sure what you mean by deriving an ECDSA public key from a MultiChain public key.

But perhaps you mean from a MultiChain address? If so I'm afraid the public key cannot be derived from this, since the address is the text form of a (one-way) hash of the public key.