(osso- Thank you for the move.)
Thanks all, panic temporarily averted.
Actually, my primary consideration is originality; as the Spider is a preservation car, even the headlights are the original 45/40 watts. Like I said, dim bulbs. Plus, it's a nice, soft look, sort of like the car.
LED lights would be way too bright/harsh, and though I have some spare NOS tail-light lenses, they're way too full of pigment, they'd look out of place; it wears the originals that are moderately bleached, but not deformed or crazed. The swirls are great. And carl750 is right about heat; I'd be afraid LEDs are way too hot for old plastic.
Another consideration is electrical; the Marelli DN22 single field generator, also fit to the early Alfa 1900, puts out a whopping 200 watts. When you add up all the components that could be in use at the same time, such as a variety of lights, Mousebreath™ heater fan, wipers, etc., well, it gets real close to it's limits. If I changed all four corner lights to 21/5 I'd be adding 12 watts to the load, 6% of capacity. Doesn't sound like a lot, but when you're close to the edge...
I suppose if I get truly desperate I could try adding resistors, but I'd really like to avoid the mickey mouse. Digging through my shelves yesterday I was lucky enough to find a pair of NOS Aurelia B10 sedan front turn signals, of which one still had it's "3/20"W bulb! I may have used the other one already, years ago (I really don't remember, I thought they were empty). I should test it to see what it's really rated at (carl750, can you point me to/supply me with Osram's test info? Thank you.), and if I can, find some way to test it's luminosity before I put it in. At least then I'd know what I might someday be trying to match.
So, I'm good for the moment, but it's just a matter of time before another burns out, so I'm still looking. Just not quite as desperate...