What sort of engine is compatible with this car?Engine swaps are the biggest power gains.
Fast road cams, adding forced induction and remapping.
Think of tuning as adding a % of power to your car.
If you start with 200bhp and add a fast road cam you'll gain around 10% giving a decent 20bhp gain for your money.
If your starting point is 70hp and you added a fast road cam you'll only gain around 7bhp which won't feel that much different and it will cost as much as on the higher powered car.
Instead on a small engined light car I would focus on improving the handling, with suspension and brake upgrades. Lose any extra weight you can and the car will feel faster and handle much better.