The RTGs on the Voyager spacecraft weigh 38kg. They initially produced 160 Watts. That sounds like a no-go to me.
Other hand Pu-238 produces 570 W/kg. A piper cub with it's 48kw engine would need 87kg of Pu-238 to produce the same energy.
I looked up the cost, $1968/g. So 48kw would cost $171 million.
Interesting idea but doubt.
Presumably this is why Pu-238 is used for RTGs where power density matters; a half life of 87.7 years means a reasonably long working life using with only 1/3% of the mass compared to Pu-239.