Most common problems on Miatas are a bad coil pack, Cam Position Sensor, or the Crank Angle Sensor.
If you've recently changed the timing belt, it is a common mistake to place the timing wheel on backwards, which will also prevent it from firing.
I would suggest the Mazda Miata dealership site which is www.mmmiata.com, another site to try for Mazda Miata parts is www.autopartsdeal.com/Mazda/Miata/parts.html. For used parts you can try www.autoguide.net
I have replaced the coil and distributor cap.
Unfortunately, a 1997 Toyota Supra engine and trans would not fit into a 1990 Mazda Miata. The Supra engine and transmission are unable to be mounted correctly within a Miata to make them fit.
Dirty fuel injectors or spark plugs
It would have to be the mazda Miata, then the Honda s2000
plenty of things could be a starter, alternator, ignition switch, bad spark...
The places to purchase Mazda Miata parts are many and varied. The best place to start would be Go Miata. Failing that a good option would be Auto Trader or the auto section of eBay.
A used Mazda Miata is a quality car. If the miles on the car are fairly decent then there is no reason why it would not be better then a new cheaper car as it still looks good.
Independent rear suspension.
You should never need to replace the wires leading to the spark plugs. The 2003 Miata has a coil pack at each spark plug (inside the spark plug hole). The wires leading to the coils are low voltage and should never break down.
That would depend on the year and how you drive your Miata. In my 1990, I drive it hard, and get about 26 miles per gallon city and about 37 on the highway driving at a constant speed.
There could be a number of things causing this, two that I am familiar with are either the timing may be off or you could need new spark plugs and wires. I have a '94 Miata and mine started to miss not long after having my water pump and timing replaced, so I took it in figuring it was the timing belt, but it turned out to be the latter. It's not an overly expensive fix if that turns out to be the case, so there's no harm in looking in to it.