If you can open the boombox, try to measure the belt length with a cotton string, put it that way the belt goes. Not easy to explain in English, with the length, or diameter of the cotton string minus a few percent (like 3- or round about 5 millimeters) you get the original length.
When the new belt is inside, wait a day because the belt stretches slightly (sometimes, let the drive run for a few minutes), than you can set the speed more correct. If you have NO idea to set the tape speed, use an original Music cassette and play the song and also the same song with your computer and try to sync it. Not easy, i know, so the song should be a little longer... Now you have to set the tape speed in sync with the song that plays on your computer (please use a computer, with the space bar, you can pause and play the song precisely (-:
The easy way would be to record a sine tone with you best tapedeck (500 or 1000 hz) and load an oscilloscope app to your smartphone.