Well you are going to have a hard time finding things for the stratus right now. But just wait a few more moths and they should have a lot of stuff for it. For now just check out all the normal places such as;
www.borla.com www.stratussphere.com i think xenon might have a body kit for it.
As far as the engine the stratus engine is the same as the eclipse but the amount of room is different. internal stuf will fit ok but external dress up or performance may need some custom work. If you really get into you can have just about anyone make you custom parts for it.
So for your best bet just wait it out it seems to be a hit so far.