I've checked out your interior & trunk threads...just buy a new tank! You've done such a clean job on the rest it's a shame to have yellowed/stained stuff under the hood...don't just paint it.
IMHO, clean new windshield washer & coolant reservoir plus stock decals will make your engine bay look factory fresh. I think it makes any CAI & other bay mods look even cleaner.
Also, we in the motorcycle world use a product called S-100 that you spray your engine & wires to give them a clean black look. Its water repellant and lasts a long time. You can also use tire foam...it gets into all the nooks & crannies. Just wait a day or so to hose off the compartment, then do a thorough wipe down. NOTE #1: I use a leaf blower to dry the motor with it running. NOTE # 2: I always stuff rags above the Optispark to prevent intrusion...don't forget to remove
There is probably a thread on here that covers engine bay cleaning but that's how I've been doing it for years and it works for me.