I have a 95 g103 with the same or similar engine. I have also found no workshop manual but at 90 bhp stock Id reccommend a couple of breathing aids i took the 'airbox' style filter system out completely and put a pipercross off a punto of it. The exhaust on the car was broken at the flexi part when i bought mine so i suppose id reccommend a straight thru from the cat, failing that bypass the cat altogether and have a straight thru pipe to the back of the car. On my own car i can pass nearly any 2 litre saloon on the motorway and the car will out accelerate a vtec civic... i think maybe 130bhp? itll do 120 mph and thats the clock so its good enough for me.
It goes without saying your engine should be serviced regularly and be in perfect working order and your brakes and suspension up to speed too, otherwise all mods will be in vain