Chris, hoping you can tell us (!), the bitumen/felt panels seem about impossible to find, maybe more-so in the US than in Europe. Hardest to match seems to be the fully saturated panels on left/right of the engine bay, and the choice you have here are about as good as I have seen for carpet underlay. It seems that bitumen in the interior even became illegal in some places (Australia). I've seen some Mercedes/Fiat places in Europe that hint at having near-misses of bitumenfilzmatten. But otherwise, bitumen seems to be targeted at big orders for construction, I could not finding a friendly source in the US.
Yes, the heat panels are white asbestos under an aluminum frame. I ended up using solid aluminum panels only---getting the original look on the centers would be an exceptional touch.