Ok where to start. A few more pics, sorry I didn't take enough. There's a little bit of a jump here, lots of decisions made using what I had at the time. At the time I was also in a mix of doing everything at once. Thanks for your comments, you guys are great!
I haven't really checked the posts recently after my last one, but some of the suggestions are getting close to what I did.
I decided to place to switch inside the tail by cutting a square hole, fitting the switch in and using green stuff to mould some of the scale back over the switch outer case. I've also only glued 2 side-reflection-panes on the engine, rather than the crystal holder, which I've scraped.

What of the switch you may ask? Well I found a use for the optional tail scale:

First I made a square indent in the scale to fit over the switch. Then carefully drilling through the scale into the switch, I was able to pin them together. Because the scale moves to turn the switch on & off, it was easy to see part of the switch under the scale. So I unpinned the scale and with the help of some more green stuff I extended it. It's able to move without noticing the switch, it's as if the switch were never there.
Also there^ testing if the Arc piece fits into the engine (with some cut up spare parts and cutting away) with the optical fibres placed inside and through a hole in the Basty shell.
Massive leap in progress here, I did manage to scavenge a couple of clear plastic crystals off someone but they're smaller than planned. They come pretty close to the suggestion by pendrake, but are a little pointier.
PENDRAKE SAID: ↑
Edit: Is Amazon an option?
http://www.amazon.com/Small-Clear-Crystal-Like-Ornaments/dp/B004ED7H3M
To also answer your questions
@pendrake the original crystal in about 15mm max wide. I did a test with one crystal by drilling into half of it, putting a 3mm yellow LED in it and connecting to 3v. The whole crystal lit up! Better than shining a light onto it. So the plan was to make that one the front crystal; and for the main crystal I would shine LEDs from the underside and rear like the drawing.
In the pic the Solar Engine is just about ready to assemble having used a lot of cutting & gluing and trial & error to make sure the LEDs fit inside the parts.

Left to right we've got:
- LED for front crystal, and underneath main crystal (the black tube is heat shrink to stop light escaping out and under the Solar Engine)
- Front section to hold small crystal (front LED goes through hole into front crystal)
- LED to shine behind main crystal (to be placed inside Arc piece through hole in rear-mechanism-part-thing; why did I place it over there in the photo?)
- Holder for main crystal (placed in centre of Engine for LED to shine through; made of a couple of plastic gears glued together)
- The Solar Engine
- The Arc (fibres have been glued to the 4 square tubes and then other ends all collected & cut flat to sit against LED that is also wrapped in heat shrink with them to hold them together; not sure if this needs another pic or diagram)
- Main crystal (not as big as expected but the shining LED from the rear does line up with the side corner, which I think lights it better when positioned there)
- Front crystal (not as pointy, buy hey, you'll see it light up soon)
So when assembled it looks just like my diagram, but the LED in main crystal is pointing at rear of the crystal.