Wing Mfg. used to have foam cores for the Phoenix series.
I would check their website or email them.
I am sure you can get replacements.
I would try the "Classic Pattern" site as well.
I was a big fan of the Phoenix series when I competed in pattern back in the early mid-80's
I had a P-6 and P-8.
Your model does not have the chin characteristic of the 8.
It looks as though your wing skins wrap around the leading edge of the foam...there is no shaped balsa leading edge.
Is this correct?
I'd sure bet it was a 6 but I have been wrong before.
The only reason it is important is so you get the right wing.
JLK