If a generic audio cassette will play back through the Porta-One, that might work too. Even if some tracks are playing backwards (I'm not familiar with the Porta One, assuming it takes the same 4 tracks that are standard on all cassettes, but plays them all four in the same direction, rather than two forwards and two reverse like normal cassette players), you can still tweak the playback head for maximum high trebles (sibilance, etc).
If you get the playback head aligned, you can then align the record head by playing back one track while recording another into the mix. This time, tweak the record head for maximum high trebles. If you have a signal generator, it works even better to use a high frequency test tone (sine wave) as the record signal, and maximise its playback volume by tweaking the record head alignment.
You probably know that head alignment is particularly critical for cassette players. Because of the very slow tape speed, recorded wavelengths on the tape are extremely short. If the head is even a tiny bit out of alignment, the shortest wavelengths (treble) won't line up with the head gap, and you get drastic treble loss.