There is not a direct link between APDIST/APMAST and POTRAN/POMAST. I mean, there is not a field in APDIST or APMAST that gives you the PO# related.
This is one modification you need to work on, addding the field PURNO in APMAST/APDIST in order to get a complete report easier.
Once you added this field, you need to fill it up when receiving Unbill POs in AP.
Now, I understand that your current situation is you have not this information in apdist, try seeking the PO# in the APMAST file, maybe it was entered by accounting people as a reference in the field APMAST->REF.
If you have not a APMAST->REF field helping you, you will need to get the file ICTRAN involved. ICTRAN has the details about what you are invoicing in AP. This file also stores the vendor# in the field ORGNO and the PO# in the field DOCNO.
So try to do your best to get your report.
My final suggestion is to add the field PURNO in the APMAST and update it when an unbill PO is save.
Hope it helps you.