It depends on your power goals. 200-220 is going to be a nice, fairly safe range on a stock motor. A 2560 will get you out to around 250-260 which will be on the upper end for a stock motor and drivetrain.

 

If you are in a part of the world with very few 1.8 engines, the 1.6 will be fine with the little Subaru turbo. It will spool well. Save the money you would spend on importing the 1.8 engine and instead make sure you get the differential and other supporting modifications for reliability. 200 wheel horsepower, or 220-230 engine horsepower will be easily attainable and will be great fun.

 

One thing to note is you can find a nice 1.6 for cheaper than a 1.8 usually. If both are stock, and same mileage/condition, both would end up needing the diff dropped for bushing or seals ect. At that point, upgrading the 1.6 rear end isn't a big deal except for added cost, that you may have saved on in the choice of buying a 1.6 in the first place. 1.8 is the better option to start from of course being a bit more ponies, slightly bigger brakes and the rear end is already installed, but IMO it's not a deal breaker unless you are strictly focused on building a race car. I actually like the 1.6 more partially do to it is the underdog, plenty of parts for sale because "no one wants them". I think it sounds better. It all really depends on the specific miatas available to you, sometimes 1.6's are in better condition.

 

Yep, I just hopped on https://www.autolinkmx5.com and found a differential as you adviced for a fair price, now I just need the axles and driveshafts

 

If the 1.8's are harder to source, you could also run the current 1.6 motor NA and learn to tune with an aftermarket ECU while building a 1.8 motor on the side. This way you can buy turbo parts for the 1.8 and not have to do everything twice.

If you're going to be content with 220ish then the 1.6 and subie truboz is gonna be a good option. They scoot at that power, especially when the boost and torque build early on.