I need help finding freeze plugs to complete the engine rebuild and have had no luck with searches. it looks like I need 16mm and 18mm brass but cant seem to find a place to buy them. any help would be greatly appreciated.

from another Duc forum:
"87310201A 20mm
87310051A 18mm
87310011A 16mm
87310021A 10mm

If you type in Ducati then the part number (without the 20/18/16/10 etc) you should find a few that sell them."

I assume shops that build engines or shops for agricultural machines should have these, too.

This is a great site for parts cross reference: http://www.desmoduck.com/index.php. Just copy and paste the part numbers into the last field and you get an overview in which newer models these parts were also used.