I assume the space.com story refers to a shortage of plutonium
for use in RTG's...The same type of RTG's used in the Voyagers,
Galileo, Ulysses, etc...

RTG's were never really efficient and required a "large" amount of plutonium.

What about the Stirling-Engine?
I understand that it's over 20 percent efficient in converting
heat energy into electric power.

A Stirling-Engine can use sunlight, radioisotopes, or any suitable source of heat
for power.

Furthermore, it can use less plutonium than a RTG to generate
the same amount of electric power.

Ironic, isn't it?
The Stirling-Engine was invented about 1816 [Napolean Bonaparte was still alive]
by Presbyterian Reverend Robert Stirling.
Good thing Napolean's army engineers weren't the ones to invent it a few years earlier;
he might have won Waterloo with the aid of some self-propelled war-carriages.