new Api - very similar - you just have to find the template based on id or alias

 // connect to the contentTypeService 
IContentTypeService _contentTypeService = ApplicationContext.Current.Services.ContentTypeService;

    // get the docType 
    IContentType docType = _contentTypeService.GetContentType(id);

    if (docType != null)
    {
            // get the template you want to set. 
            ITemplate template = ApplicationContext.Current.Services.FileService.GetTemplate("myTemplate");
            if (template != null)
            {
                    // set 
                    docType.SetDefaultTemplate(template);

                    // save
                    _contentTypeService.Save(docType); 
            }
    }