We get asked this question quite a lot so here is an introduction.
Feel free to add your suggestions and pointers below.

1) Please read these articles
How does a Catalytic Convertor works2)Notes:
Please note that I don't think it is worth modifying an old car with a small capacity engine.
There is nothing stopping you but the money and hassetsetsetsle will just not be worth the
power gain you get. It is better to just buy a better car for your project.
Feel free to talk here.