I can't cast any light upon the matter of the spark plugs, but I would be
surprised to find Po210 as the polonium isotope of choice. That isotope
could be milked from uranium ore, but a more substantial quantity of
another isotope, Po208, could be milked from thorium ore (parent nuclide
Th232). Po208 has a longer half life, almost three years.
When I do demonstrations involving a radioactive source in lectures (at
least when I'm not radioactive myself) I use a vintage Coleman lantern
mantle which contains a lot of thorium.