Funny, I have been contemplating making one myself. I already bought some spring loaded clips with test leads attached. I already have spare motors, light bulbs (and as an alternative LEDs and resistors). All I have to do is to put it all together in a box.
That's all you would need. You might want to consider just building one yourself. It would be as difficult as installing a wired decoder in some locomotive, as it is basically a locomotive without the running gear (motor and couple of headlights and few extra lights if you want to be able to test more functions than F0F and F0R).
