I thought to put some links and info together regarding performance parts for the N/A foresters.
SubaruWRXparts.com: Quality OEM and aftermarket parts for your Subaru Impreza, Legacy, and Forester.
Subaru Performance Parts, WRX, STi, Forester XT & More!
Tech Works Engineering is Subaru performance, upgrades and aftermarket parts
On naturally aspirated subaru, there are four sections to the exhaust. Starting from the front of the car, there is the exhaust manifold (or header), then the cat pipe (stock cat pipe has two catalytic converters), mid pipe (stock contains a resonator), and the axleback (muffler and piping).
Headers/Cat-pipes and mid pipes are interchangeable between 93-01' Imprezas and 98-02' foresters. Exact same pipes, meaning any aftermarket header, high flow cat, or mid pipe for the impreza will bolt right up to your forester. Axlebacks are a bit different, as the impreza is shorter, so you may have to use XL exhaust hangers or get the tip extended to come flush with the bumper. Stromung makes a forester specific muffler, and some people are modding WRX and STI cat backs to fit. I would stay away from WRX aftermarket exhausts on an n/a subaru, as the piping is too big and you will lose power/torque. *06+ N/A subies have a different header/catpipe set up. The older style headers and HFC pipes will not work on 06+ subarus*
Intakes from similar years N/A imprezas will fit. Direct bolt-on. Just be sure that if you have a 98-99' forester, you use an intake designed for a 98-99' impreza. This is due to the MAF (mass airflow) sensor found on the 98-99' models. Model years 2000-2004 use a MAP system, and you can make a DIY intake for pretty cheap. The MAF system started being used again in 2005+ models.
Lightweight crank pulleys are pretty much universal across the whole subaru line of cars (including turbo models).
Upgraded camshafts and other internals are just the same as parts for the N/A impreza (same EJ25 motor). Just be aware that 98' foresters and a small number of 99' Foresters had the DOHC motor. The rest have the SOHC. TWE makes cams/internals, and Delta Cams can regrind your stock cams to be more aggressive for a decent price. Untitled Document
Rallitek offers a PerfectPower6 (PP6) piggyback ECU for N/A subies, allowing adjustments to be made with timing and air/fuel mixture to get the most out of your mods. I-Speed also offers ECU reflashes for stock motors, or with different stages of mods. I know there is at least one member on here with his SF ECU reflashed. I - S p e e d U S A
RomRaider (open source tuning) is also available for 2005+ N/A subies. Info here: RomRaider - Open Source ECU Tools | Documentation / Rom Raider FAQ
Cobb tuning released an AccessPort for 2005+ N/A Imprezas through Rallitek. Currently they do not have forester maps or support, but with enough interest, they easily could start supporting the Forester community. I encourage you to email Rallitek and let them know that you're interested. http://www.rallitek.com/acimmt.html
If I forgot something, or you know of something that should be added, please reply and I will add it.