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.
Put on a set of twin 40's for a fairly simple power hike. A few guys are dropping in Coswroth engines to these cars, they have nice big engine bays and are RWD making it a fun car indeed.
If you want to go to the max then cryo treat the block, get it rebored and look into blueprinting and balancing as well.
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 - a 4,2,1 branch should work well.
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.