Hello there and may I wish you a very warm welcome to TorqueCars forums.
It is nice to meet you. I hope you enjoy the forums as much as we all do and that you get totally addicted to this site.
The best mods for a car engine are as follows.
Internal work yields the biggest gains, head work like porting polishing and 3 angle valve jobs will maximise the airflow and raise the power.
Bigger valves are an obvious addon but not all engines are suitable as it depends how close together the OEM valves are.
A fast road cam will yield good power gains, especially in the top end of the rev range. Don't go mad though a profile of up to 270 degrees will give the best gains without the engine becoming cammy.
The most popular power mods are induction kits (which are questionable as power mods but they can sound nice if that is your sort of thing) and sports exhausts with sports catalysts or decats.
To keep the car legal I'd recommend a sports catalyst rather than a decat.
It's worth having a read of the articles on
www.torquecars.com/tuning/ for more information on these mods and what the terms mean.
I'm sure the 1.8 Mazda engine has been used in other cars, it might be easier to source compatible parts that way. Is it the same block as the MX5?