How to tune your NA engine with "bolt-ons"

Sorry, this is written as an answer to a question, but alas, it’s too long to fit in a comment field. So the writing is directed at one person, still everyone wanting to tune NA without opening the engine should benefit from this.

Put the cat back in. Should give you more power. Reason? Especially NA engines depend on a bit of backpressure.

Other than that, build an intake manifold with a brrel like plenum and have equal length runners go into the head. Don’t make them too big in diameter, otherwise you’ll lose air velocity and thus power in the lower range and unless you modify the RPM Limiter who knows if you ever get to high enough RPM that the high diameter will really help your engine.
Install a Throttlebody a bit bigger than stock (not too big, see above).
Shorten the intake piping where possible but still keep the actual open nozzle in front of the engine, away from heat and with a relatively direct airflow from the outside. But make sure no water can get into it.
Get a freer flowing intake air filter, if you have the space get two and install them via a Y-joint. But keep them away from heat and moisture. A simple K&N replacement panel filter can already give you a bit more power and is rather cheap. Also, because of people not shielding their airfilters from engine heat or installing them in a way that rain and moisture can get to it during driving, most of the time a replacement panel filter gives more power than the cone-type.
Try to get the first 10 or 20cm of your intake into some kind of funnel shape, decreasing the diameter until you are at a diameter a bit bigger than the combined diameter of your intake runners. This will increase air velocity and thus increase bottom end power a bit.
Heatshield the intake piping from the heat of your engine as good as possible.
If possible, build a dual-runner intake manifold, similar to Toyotas “ACIS” system with smaller runners for lower end and bigger ones for top end. BUT you have to get the ECU to understand that. It’s similar to VTEC, just not integrated into the head and can be combined with VTEC or similar system (VTI-S, VANOS etc). This one thing is, when the rest is already there, the single one most beneficial bolt on mod, equal to changing camshafts but without having to sacrifice either lower end or higher end power.

For the exhaust, build a 4-2-1 exhaust manifold, make sure all the runners are equal length (unless you like Subaru sound, then make one of the runners significantly shorter).
Reinstall a Cat, but use a 100 or 200cell one. Better for our environment and our health (I’m no greeny, but a cat really doesn’t hurt power unless it’s clogged, you are running significant boost or it is way too restrictive).
Make sure the exhaust has roughly the same inner diameter over the complete length. Also install only absorption style mufflers. (Not no mufflers, as then you will really dislike driving long cruises and you will get the hate of nearly everyone you pass by)

Get a lighter flywheel, this reduces the power the engine needs to spool up but you will get problems when going uphill. Also use lighter weight pullys all around.
Get lighter rims. Apart from rims like the OZ Superleggera, a general rule of thumb is, smaller rims are lighter and steelies are lighter. This will give you a better acceleration, as removing weight from the rotational gear, including rims, gives you at least 10 times the benefit compared to removing weight of the whole car. 1kg lighter rims equate to at least 10kg in removed car weight in terms of acceleration performance. By going from for example 15” alloys to 13” steelies you may save 6kg on wheel-weight all in all, maybe even more, depends on what you have on now.

And the last thing, other than opening the engine and make internal modifications, tune your ECU to cope with all those bolt on mods, otherwise it will run badly in most cases.

Btw, this list is suitable for EVERY NA engine (at least with EFI, I would think for Carbs, too)