After searching all over this forum & the internet, I was unable to find the correct Piaggio water pump seal drift that would allow setting the seal correctly.
A custom machined (not available) drift can be seen here https://www.youtube.com/watch?v=RYj6gr33yaQ&t=1876s
For those who don't know, the seal has a tension spring and needs to have the outer half and inner half pressed in place at the same time but at different depths. You can't just drive the outer half only without destroying it. The spring needs to be able to compress in the drift without bottoming out. The seal is very fragile, and I destroyed one before I came up with the method below.