Ok so I now have something working, feels very dirty though.

        private void EditorModelEventManager_SendingContentModel(HttpActionExecutedContext sender, EditorModelEventArgs<ContentItemDisplay> e)
    {
        if (e.Model.ContentTypeAlias == Alert.ModelTypeAlias)
        {

            var rootId = e.Model.Path.Split(',')[2];
            var siteRoot = e.UmbracoContext.Application.Services.ContentService.GetById(int.Parse(rootId));

            if (siteRoot.Name.ToLower().Contains("site2"))
            {
              //we need 
              //will probably also need to fix up audience in grahpql??
              var property = e.Model.Properties.FirstOrDefault(f => f.Alias == Alert.GetModelPropertyType(c=>c.Audience).PropertyTypeAlias);
              var datatypeService = e.UmbracoContext.Application.Services.DataTypeService;

              var all = datatypeService.GetAllDataTypeDefinitions();

              var myAudience = all.First(x => x.Name.Equals("Alert Audience"));

              property.DataTypeId = myAudience.Key;

             var dataType =  datatypeService.GetDataTypeDefinitionById(myAudience.Key);

             var preValues = datatypeService.GetPreValuesCollectionByDataTypeId(dataType.Id);

             Dictionary<int, Dictionary<string,string>> items = BuildItems(preValues);

             property.Config["items"] = items;

            }
        }
    }

    private Dictionary<int, Dictionary<string,string>> BuildItems(PreValueCollection preValues)
    {
        var newDropDownItems = new Dictionary<int, Dictionary<string, string>>();

        foreach (var item in preValues.PreValuesAsDictionary.Where(k=>k.Key!="multiple"))
        {
            var values = new Dictionary<string, string> {{"value", item.Value.Value},{"sortOrder",item.Value.SortOrder.ToString()}};

            newDropDownItems.Add(item.Value.Id,values);
        }

        return newDropDownItems;
    }

Works saves fine and when you view item in backoffice it restores value correctly. I am having issues on frontend which in my case is graphql may need to tap into the render with a resolver.

Regards

Ismail