It depends on how old the car is, your skills and patience, how much money you have, time and resources.
A car from (say) the 1960’s will be easier to work on, as it has few, if any electronics to worry about. It’s all mechanical & analogue.
Applying the ‘upgrade’ techniques that would have been applied back in the day will still achieve the same results.
So, greater displacement, higher compression ratios, hotter cam(s) (greater lift, duration or overlap), bigger carb(s) porting/ polishing… etc., will all result in more power, though often at the expense of much greater fuel consumption and poorer driveability. (Hot cams can make an engine a complete pain under a few thousand revs, & the idle will be atrocious!).
Move onto a car from the late ’70’s to early ’90’s and you’re into fuel injection, without the likelihood of complex security systems and everything ‘talking’ to everything else with the need for coding…
Using similar techniques to guys in the ’60’s, you can get increase in power too & remaps can optimise fuel delivery & spark to the new engine spec. But is isn’t necessarily something you can do yourself, so expect to rely on someone with a rolling road to get the best out of your vehicle.
Car’s from the mid ’90’s onwards are into the realm where you need to ensure that the various electronic modules can communicate, otherwise you can find that your modifications result in a car that won’t run.
Example: I fitted a manual gearbox to an E39 BMW 540i automatic. Getting it to run was easy. Getting the traction control, dash, ABS, and even the reversing light to work properly took a lot longer than swapping the mechanical bits..! Save for one wire, it was all coding & before all that was finished, I had to leave the autobox ECU (for the by then non-existent autobox..) in place, otherwise it wouldn’t run!
Also, bear in mind that many ‘upgrades’ done to older car now involve fitting modern (digital fuel injection) systems to older cars. The bang per buck (power!) achieved with a modern drive train can far exceed what could be achieved using older ‘dumb’ technology.
Read more about this topic at Qoura
Never do it to your car