I did this quickly, not using the right sort of casting plaster, force drying it, skipping much of the finishing process, etc. As a result the outcome is a bit rough, but works as a proof of concept. Although the shape in the pic looks a bit distorted it's perfect (the distortion is the camera angle/lens).

If I did this again using the right casting plaster and properly finished the mold, I could make a virtually exact copy of the original. I cast it out of epoxy resin with a clear red colorant. I'm thinking I might make a pair of clear lenses and use red LED's instead of bulbs. Ought to be a good deal more visible, if a bit obnoxious. I might also add a nice pair of interlinking M's to the mold for the Morris Minor bit.
If anyone wants details I'll write up the process with pics. The cost of all the supplied/materials would be cheaper than a pair of new lenses even if I didn't have the stuff around already. The application here, of course, would be for lenses that are no longer obtainable -- which fortunately doesn't include us moggie folk.