patch for cracked plastic shield on roof?

line, on the side facing the top of the roof (i.e. in the path of water/sn ow coming down); how should I patch it? It's about 1/2" wide crack, extend ing up the 4" height of the plastic shield. Maybe just plastic roof cement and yellow roof fabric? Or maybe this "FixAll" patch for $8 at HD:

I like the HD patch. This is a good example of why these things should be made of aluminum, not plastic. Might be time to consider replacing it, depending on what the consequences of failure are. For sure if you leave it there, you have to keep an eye on it. If it's cracked, it's probably going to crack more.

Should I remove the screw and fill the hole with polyurethane caulk, or jus t leave the screw and put roof cement over the screw head? (Replacing the shingle is not an option.)

Either is probably OK. I'd probably go with taking the screw out, then lifting the shingle, getting roof type caulk, poly or silicone, over the hole, then finish with a dab on top.

Flex Seal for both?

I don't know how old the fan is but I am certain it will only get worse. If you choose to repair it the repair will only be temporary. The only way to do it right is to replace it. I would hope when you do replace it you opt for a higher quality component so you won't be doing this again in the hear future.

My fan is 30 years old and it's fine. It does have metal flashing, a metal dome, and a metal blade. No plastic at all, I think. I've replaced the motor, from the inside of the attic, 4 times. Once it lasted 14 years, once only 3. Other times in the middle.

Last year my trailer vent had a hole. I used duct tape. Still holding.

Greg