You can use a metal razor to do the work, but use it perpendicular to the work. You still need to be careful with it on aluminum because it will cut into it if you're not careful. Using it perpendicular to your work surface allows you to scrape the gasket off without cutting into the metal as you very easily will at an angle. Doing it any other way, even for a practiced hand, is surely a good way to ruin the mating surface, even if you're uber careful (ask me how I know).
A plastic razor (gasket removal tool) works okay and won't mar the aluminum surface. It won't work nearly as well as a metal blade, but it is a lot safer on the mating surface.
Do not use Scotch Brite pads on aluminum. It WILL cut into the metal. Scotch Brite works wonders on cast iron without the fear of causing issues, but can easily cut into aluminum.
To clean it, the best thing to use is acetone. If that's not available, IPA or brake/carb cleaner would be good. Even WD-40 is a good cleaner (that's what it was originally designed to be), however, you'd want to ensure you get it fully cleaned off of the mating surface as it may interfere with your new layer of sealant. I'm sure Seafoam would work just fine, but it seems like an expensive way to go for a surface cleaner.
EDIT: One other thing, if you have fingernails, use them to see if your work surface is actually clean. The tip of your fingernail can tell you if what you're seeing is leftover gasket or is just a stain on the metal you don't need to worry about. Looks can be very deceiving. Your fingernail will tell you where the truth lies.