a 3d printed flame would work well and is achievable.
You could design it so it is available from Shapeways along with a heap of other pinball parts and mods and they have a material that is excellent for shining light through - detailed plastic. It is opague but put a coloured light behind it and glows up nicely.
here is my tail light mod for the creature which there is a detailed plastic lens in it which has a white led behind the pointy bulb section and a red led behind the flat brake light section.
here is the site if you are curious of the possibilities.