Roof Venting Repair

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I looked to repair a metal roof today that has three old worn out roof vents where two of the tops had blown off allowing a lot of water in. They are the "spin in the wind" type about 12" diameter. I am considering suggesting to remove these vents and installing new ones but am not sure what type to install on this metal roof. The attic is 38' wide, 85' long, and 9' tall in center. I also will add more venting to the roof at the same time.
Also there are some 2" rubber boot flashings that are cracked and need repair. How do you guys deal with these on a metal roof? Replace whole flashing? Alternative?
Suggestions?
Thanks,
Dave

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Missouri Remodeling Company with services for Replacement Windows Vinyl Siding and Entry Doors