What you have to do is....
Keep the original cassette you recorded on in the porta studio.Get the mix sounding the way you like it.
Then connect your porta studio to a regular cassette deck with an RCA cable.coming out of the master l/r
jacks on the back to the tape in on the cassette deck.Push play on the porta and record on the cassette deck and voila.
A thing about porta studios is you cant just record on them and use the cassette you recorded on to play back on a regular cassette deck.You have to make a stereo master from the original tape.
Hope this helps.