Joe AutoDrill writes:

If I buy magnesium from somewhere (McMaster doesn't seem to carry it
or I'm semi-blind), can I do the same thing?

Sure. I've done it scraping the tubular handle of a lightweight magnesium
landscaping rake. Odd chunks are on eBay all the time. I have 1/8"
magnesium welding rods from aerospace surplus.

I'm skeptical of the engine block tales. Large pieces don't ignite since
they have little surface area. The 1/8" welding rods self-extinguish if
you light them. They'll only maintain burning if you hammer them into a
flatter ribbon shape. And weren't the engine blocks an Mg-Al alloy?