Hi
I Looked at your link
Class 31 Diesel
If the lights are molded in the Triang Class 31 body
you could drill them out with the appropriate sized bit
and mount LEDs through the holes , provided you can create a bit of room behind the light fitting for the LEDs and wiring
LEDs come in various shapes and sizes and can be modified to fit. The ends filed flat to look like a headlight Lens
Fiber optics is another way to go , The ends of the optic fiber can be heated to create a lens of the correct size
the other end of the Fibers brought together to a LED
Hope this helps
Regards
Zmil