But this guy was asking "short of an engine transplant".
So to answer his question, I would say: extractors, 2" exhaust and cold air intake system all combined will give you a bit extra. Also remove any weight you can and make sure you run small lightweight wheels and tyres. Lightweight is an excellent modification to go for. From here you could get an aftermarket ECU and tune it better, raise the rev limiter, larger throttle body, cams. All of that combined will get the car moving along nicely.
But will definately cost a lot more than slamming in a 1.3litre engine. 1.3 transplant is definately the best bang for your buck as everyone is saying.