I have used total recorder in the past to convert books on tape. Total recorder allows you to record for xx amount of time. So for 90 minute cassettes I set total recorder to record for 45 minutes, start the tape, and let it record as a wav file. I come back later and use total recorder to mark the beginning just before the sound starts and mark the end just after the tape ends and do a SaveAS to eliminate any extra silence at the beginning and/or end. I then convert the wav to mp3.

Since your player can play both sides, I would record the whole thing then SaveAs twice, once for each cassette side.