I've made recordings of the two cassette tapes. The Koei tape had data on both sides. The shogi game just had data on one. The waveforms look pretty clean to me, so hopefully they're still good.
I'm going to PM Kale with a link to the recordings, plus scans of the tapes and related material (the shogi game also had a tape cover and instructions).
As first, I am absolute not sure, Paul.
If the KOI .wav images (PC 6001 MK II) are basic games, maybe they are in wrong .wav format.
I can not convert these .wav images with P6DatRec.
Have a another .wav image (Lode Runner) as reference converted to .P6 which is working fine on some Emulators.