public class SampleApiController : UmbracoApiController
    {
        private readonly Lazy<IContentResolver> _contentResolver;

        public ContentApiController(Lazy<IContentResolver> contentResolver)
        {
            _contentResolver = contentResolver;
        }

        public IHttpActionResult Get(Guid id)
        { 
            IPublishedContent content = Umbraco.Content(id);
            var model = _contentResolver.Value.ResolveContent(content);
            return Ok(model);
        }
    }

// Converter:
public class SampleConverter : IConverter
    {
        public string EditorAlias => "My.PropertyEditorAlias";

        public object Convert(object value)
        {
            // If the value is already in a json supported format, just return it.
            // Otherwise convert it to a friendly format here.
            return value;
        }
    }
// Composer:
    [ComposeAfter(typeof(UmbracoContentApi.Core.Composers.ConvertersComposer))]
    public class ConverterComposer : IUserComposer
    {
        public void Compose(Composition composition)
        {
            composition.Converters().Append<SampleConverter>();
        }
    }

composition.Converters()
    .Replace<ConverterToReplace, SampleConverter>();