Yep, or do a transplant.

I would do the simple stuff like advance timing and run 98 octane (you can just do this with the dizzy ) which will make a small improvement but as others have said you don't get much bang for your buck modifying the NA.

Go for a drive in a turbo and see how you like it.

Cheers

 
If you are into engine longevity maybe stick with the NA.
Sure if power is your thing go turbo, no argument there, but keep in mind that in the Toyota service manual, many service jobs for the turbos are recommended at half the Kilometer intervals of the NA models.
Is Toyota trying to tell us something there?
 

In terms of longevity, the turbo motor is obviously under more stress.  But if you dont go crazy boosting it all up, keeping it relatively normal and treat it properly (fully synthetic oil, warm up, cool down, ear out for detonation etc) then reliability is great, that was my experience with my first MR2, and I did a fair whack of motor sport in it.  It was one of the most reliable cars I have had.  I did change oil and filter every 5000km in it.
 
Cheers

Tree

+1 for leaving it stock. I assume you want a bit more power and I can see 2 ways going around it without touching the internals.

1. Custom cold air duct from side vent to the filter - "Wiz" did this ages ago on his 97 NA and dynoed it. Got 4rwkw if I remember. He got rid of the baffle and used convoluted tubing to the vent. Coolant hose is kinda in the way tho.
2. Advanced timing + higher octane fuel. Checkout autospeed's "5 cent modification" article. Just be cautious of detonation from too much advance

You prob won't get much but it'll keep you busy for a bit :)

 
+1.  This cold air mod gets you a bit more power and also quite a lot more torque (from 186nm to 200nm in my case).  Definately use 98 ron fuel and I suggest you don't try advancing timing coz you will probably get ping in these later higher compression N/A motors ('94 Bathurst +). Possibly fit Copper Iridium spark plugs (one stage cooler), flat panel K & N filter and better exhaust with sports muffler. Probably cost around $1,000 all up.
 
Whatever you do don't get talked into buying a piggyback powerchip for the computer (like I did). All it did in my case was confuse the computer and lessen the power so I ripped it off. These probably work better with turbos.
 
All this will give you noticeable minor gains for not too much cost as Toyota have virtually got the max out of this engine with their stock ECU etc.
 
Once you go into bigger cams, forged pistons etc on an N/A you will find the car is too cranky for road use and there will be too much stress on other components and the overall cost will be ridiculous. Obviously turbo is a better option if you just want raw power.  

Lol
I wouldn't bother
Do basic mods then learn to drive the car to its limit once you achieve that then go for more power.

Also spend your money in handeljng I'd rather spend 2000$ in suspension sway bars bushes strut braces and tyres and be 20-30kms quicker around corners and on a track then spend 5-10k to have 40-80 extra kws

Just for reference
I was going to do cams 264 duration little more lift
500$
Then new belt 200-300$
Then someone to strip it and install it
600-800$
Then you will probably want to get full benefit
Port and polish the head while its off, do valves and seals, rebuild entire head... Nek minute cya later 2-3k add another 1000 for a good port and polish job
Then you still need to have it tuned properly to see any massive results so there's several hours on the dyno. My partners uncle owns a shop and not even I get free tunes.

So there's a head built and tuned for 5k

Then bottom end you would be silly if you stripped it for another 800$ to remove engine
Then just did pistons
You would while its stripped and out
Do new apr studs
Eagle rods
Forged pistons
Have the bottom end blueprinted and balanced
Even have it bored to 2.2l

For pistons alone you will see barely any result maybe 10kw
Then your changing the compression so new head gasket blah blah

Then with all that power your clutch will need to be done there's 1000$ right there
Then if not already install an LSD

So for a decent built engine and not cut corners and just do 'cams and pistons'
It will probably cost more than what you paid for the car up to 15000$
And you would probably still be beaten by a lightly modded turbo.

Then later if you decide to turbo it
Why bother when you have just spent 15k on building a hi comp engine.

Do what tree did and put a beams in it but go a redtop so much cheaper.

Just do suspension
Intake
Exhaust
Basics.

My 2 cents

engine longevity and comments on doing a turbo conversion shouldnt go in the same sentence.. just saying
if you want your engine to last dont modify it and leave it stock, toyota got it right so stick with it they are known to produce some pretty good engines that last a long time.
cheers

For a Gen3 NA, I'd stop @ intake, headers & exhaust. Cams, ecu etc is just not worth it.

I do know of a little trick the Japs do though, they put a spacer on the plenum (increasing its volume) & get a few extra ponnies. Not much, but that's cheap/easy to do. Stock ecu can handle all that.

As for longevity, 3S engines usually die from a spun bearing, cracked piston ring or a blown HG/major overheating event. All of which are more maintenance or age related. The modified ones that die, are usually due a weakness, failure or negligence.

In the case of pure longevity, there was a modified 200k+ mileage (miles no km) MR2 that was doing 11's over & over again. In Australia, we have this stupid mentality that anything over 130,000km is on borrowed time.

yup 130,000 is just run in lol
mine has still got the original unopened motor AFAIK and almost 200k and a healthy ~185 psi across all cyl.

Mine has done 230xxx
And has 185 all 4 too :)
Well looked after she was
I'm the third owner and I'm very passionate :)