Hi Chris and welcome :)
As Colin has already said this engine is pretty untuneable! You have the exact same engine as I have in my IS200 Sport, they are exactly the same across the 200 range weather you have an S,SE, Sport or Sportscross.
The only way to gain any power is by forced induction. Either a Supercharger or Turbo. Neither are easy to come by/do and neither are cheap. A supercharger, should you be lucky enough to find one as they are no longer made new will cost you £2000+ for the full kit and Turbo even more expensive. A supercharger will take your BHP from 153 to 200-220. Ive even seen them run as low as 180bhp so its a lot of money for relatively little gain but they do sound awesome and makes your 200 as quick as a 300.
To go higher you would have to turbo it but then you run up against the problem of the engine internals not being able to handle the higher boost of a turbo so you'd have to up rate them as well to avoid the block blowing up! See how it gets expensive? Oh and none of this is good for your MPG!
An induction kit and exhaust will make it sound better but give you 0 BHP gain. You also can't chip/re map the ECU so there's pretty much nothing else you can do to gain power. Keep the throttle body clean to keep the car as responsive as it can be and that's about it.
I've also found mine a bit more responsive running higher octane fuel but I can't prove that and it could just be a placebo affect to be honest.
No idea if your celica diff will fit sorry...