Avoid _id mapping

I have a legacy database with a collection with the following structure (simplified):

_id : ObjectId
Id: string
Value: Int32

And I have my C# class like this:

public string Id {get; set;}
public int Value {get; set;}

How can I map my class property Id with the Id (string) field in the collection, and not map with _id (ObjectId)?
I have tested many approaches but I usually got this exception message: Cannot deserialize a ‘String’ from BsonType ‘ObjectId’.