When the smoke cleared, she had no need for a 503 carcass, so she left it for me to use as a parts donor. Looking at my collection of 500 series slant-needle machines, I see that I have three 500s and no 503, so I think I will sacrifice one of the 500s to build up the 503 and fix some of the other 500s.
From left to right, #1 has a broken bobbin winder spring and a bad light socket;
#2 has a badly chipped nose door,
and #3 is in the least desirable cosmetic condition, with a serious paint scrape beside the stitch length lever and significant wear on the leading edge of the bed. So, it looks like #3 is going to sacrifice a bobbin winder spring and light assembly to #1; a nose door to #2 and some other parts to the 503.
However, that still leaves me short a nose door for the 503 if I don't want to use the one that is badly chipped and a bobbin winder spring. Years ago, I stripped a rusted-out 503 and kept many of the body parts but I can't find them in my mess. If I don't find those parts, I will have fodder for two more blog posts - repairing the hinge on the nose door and making a bobbin winder spring.