1) if it had the original cd player from Japan. You need to get one installed locally. Those Japanese don't work fine here. Costs would be around 15k to 30k.
2) for sensors you may visit an electrician to get it checked if there are actual sensors present and wired.
3) For tyres is totally upto you. You can use 205s with bigger sidewalls and they'll provide you extra grip and extra ground clearance as well. 195s may provide you slightly better fuel economy.
Fuel economy could have suffered due to your driving style, or maybe you've been doing shorter commutes lately, like groceries.
Try keeping the ac at 25/26?. get the air filter, oil and oil filter changed, if that's due. Check your tyre pressures.
I personally have a method to check my engines working/consumption. Where i set the trip meter to zero, drive the car @80/85 for about 10 to 15 km, and check the average. Ideally that should come out around 27 and 33kmpl