How to modify the supply voltage requirement in a circuit?
I would not trust this Mickey Mouse Design to work at any voltage unless tweaked with skill. The schematic won't work.
L1 must be connected to the Antenna to pull up the collector.
R6/R5 ratio must be increased to biased Ve to 35 mV so Vc is half the Vbat.
C4=10pF won't pass any audio.
This design looks like where's Waldo , can you count all the design errors?
Maybe it was corrected in the layout of the pCB
I suggest you look for a better design.  Does it work for you at 9V?

Looks like they tried to copy the other design and failed miserably swapping parts and misplacing others.