Broken Roof Plumbing Vent

After returning from our last trip I noticed the cap from the plumbing vent missing.  The part that the screw goes in was completely broken off.  Amazon had a lot of knock offs but only one "Ventline" like the original.  Only had it in white but turned out to be beige.  Oh well.


The vent was mounted so the broken part was at the back and the base is on an angle.  I think that this allowed more air under the cap while traveling and over time it broke due to the strain.  Plan to remount so the screw is at the front but will have to cut a piece off the top of the pipe so the cap can go down all the way. That would still give me 2.5 inches above the roof.

The Decor sealant came off easily but still need to remove the residue. Worried about scraping, scratching, or damaging the the surface with the wrong solvent. Any ideas on the best method to finish cleaning it up? Any other suggestions prior to starting the install? Thanks.