you can get strut braces front upper, lower front and rear braces, like adam says it improves the rigidity of the car. your effectively removing additional flex in a car body, too much though with big wheels and it will drive very hard and you will feel every bump, sleeping policeman will feel like driving over elephants.
i took the strut brace off my cab when i bought it, it looked nice but it amplified any bump, it made working on the engine more difficult and it caught against the bonnet. if your going to buy one check out the different ones first for fitment some are angled and bent to allow access to adjust your engine better with it on.
K&N panel filters are good, the drilling the airbox works and gives you a modest induction rasp noise, not for everyones taste. theres also a baffle plate in the air box if you remove that you can improve air flow.
Pipercross apparently do an induction kit for a mk1 the product number is on their website whether it is any good. at ?80 a tad pricey.
oh and if you hear about powerroar pipes, they are a joke they do nothing and are a waste of money at ?30.
