Restoring Horsepower on Older Cars?

Replace air filter, check and clean fuel system, replace spark plugs and spark plug wires

a big service and all things like spark plugs, filters, oils and others fluids will always help keep the engine a little healthier
to really restore lost hp I would rebuild the engine, take it apart (maybe replace bearings) and weigh all the components. the components are made with a specific weight in mind however the factory works with an error margin, you could get most of the components within a smaller margin.
a mechanic and a customer both had the same porsche but the customers porsche was a newer model with an engine upgrade, but the mechanic had rebuild and “weighed’ his engine. the mechanics car was faster, although the newer porsche of the customer was supposed to be quicker
go for a big service; oil, filters and sparks

All of the a above. ..and a remap

Fifth Gear have a good video about this

Replace spark plugs, wires, distributor cap, rotor, air filter, oil change. These all are most likely the most effective way to bring the power back close to factory specs

Bit of water injection to steam clean the engine out.

Never used it, i’ve ran water injection on my old car the more i used it the better the car ran without it. I assume it was cleaning the egr gunk from the inlet valves not checked the engine after thou.

One thing that gets overlooked that does cost a lot of power in an older high mileage engine, timing chain is stretched but also a lot of work in most cases