OK so the Stag may not be your first choice as a performance car (more a Grand Tourer) and I guess we are all so worried about other bits that there hardly seems time for this but some very worthwhile improvements can be made

Some "standard" tricks for a normally aspirated engine apply to the Stag

Head port and polish
Re-profiled camshaft
Improving the air intake

More specific to the STAG:

Tubular exhasut manifolds Gets the gases flowing nicely, +30 bhp and keeps you engine bay cooler
Changes engine sound. Have to move alternator and you lose the aircon
Fuel injection Expensive, needs a different inlet manifold
See the HRS website

One thing to consider is that the bottom end is not noted for strength and the crank can fracture

DONT FORGET the suspension and brakes