INTRO
This is the very first time I've signed up on a car forum, and I'm very new to the automotive world. I have schooling in brakes, suspension, electrical, and general automotive practices. I purchased my dream car, which I've wanted since I was very young, a 2G Eclipse GSX, and plan to create a tribute and possibly a replica of the Fast and the Furious Eclipse. It has 144k miles, and most of the work I've done up to this point has been cleaning and cosmetic touchups.

Now I've removed the cylinder head and machined it (they said it was good). I'm about to reassemble it; however, there is this strange gasket inside the coolant housing that has a key shape. I've not been able to find this type of gasket anywhere. Can anyone help?

I'll also list various questions and Hurdles I've come across down below. If anyone is willing to be my "sensei" on my journey of learning more about this car LMK I'm extremely eager to learn more. Thank you for reading, and best of luck on your builds as well.

QUESTIONS
Weird coolant housing gasket that looks like a key? Can't find anywhere.
Anything I should tackle now that the cylinder head is off?
Any must checks on the 4G63 T engine?
Have Sparco R100 seats but need reupholstering, any locations in California?
Oil filter housing orientation? What direction are those tubes supposed to face?
How do I approach putting the belts back on? timing, cam, and AC belts. (new belts)
How do I use the belt tension pulley I bought on Amazon?
Cams look scorched. Buy new ones or take it to a machine shop for inspection/repair?

Replacement parts so far (not put on yet)
Alternator
AC compressor
Timing belt kit
Water pump

New parts (Put on)
AC condenser
Intercooler
Coldair intake
Exhaust manifold
Turbo
Power steering reservoir
Spark plugs
Ignition coils
MAF sensor

PS: Most of my questions are engine, electrical, and cosmetic related because I haven't taken an engine class yet. Currently in an autobody class.