Serializing an object for two different uses in two different formats (should be simple..)

Question

I have an object which is marked as [Serializable] and it's properties are marked with [XmlAttribute] and [XmlIgnore].It's serialized for our web-site use, and it works well.

Now I need to serialize the same object for a different use (web service), in json format.
I know I can easily serialize objects with the [DataContract] and [DataMember] attributes, but I can't use both - [Serializable] and [DataContract].

What are my options to serialize the object?
Which is simpler?

Thanks a lot.