There is a great website that has many listings for used 1990 Mazda Miatas. The website is Lemonfree.com and they have several listings for the 1990 Miatas.
No. Mazda Miatas run a non-interference engine.
2001-2005 Mazda Miatas required a minimum of 91 octane.
The fuel filter for all 1990-2005 Mazda Miatas (including Mazdaspeed Miatas), is underneath the car, just before the rear tire on the left side. It will be hidden under a thin, plastic splash shield.
No, they have standard dual overhead cam, piston engines, at least from 90-05.
1990-2005 Mazda Miatas use 18" wiper blades on both the driver and passenger side wipers.
Stock gap recommended is 0.040", for Turbocharged or Supercharged Miatas 0.034" is recommended.
It is not a RWD car the only cars that are rear wheel drive from Mazda are the RX7's and the RX8's Miatas are ALL RWD.
Mazda Miatas do not have struts (they use shocks), so the braces that go across the engine bay are not absolutely necessary or beneficial as they would be with vehicles that have a strut setup. Shock tower bars on Miatas do tend to add rigidity, but there is a very mixed consensus by Miata enthusiasts on whether they offer a lot of value on Miatas.
Typically, the batteries for Miatas are located in the trunk. Because the car is so light, this helps the balance the weight of the car.
2001-2005 Mazda Miatas use 55w 9006 bulbs on the most outer positioned projector lamps. The center, high beam bulbs use 65w 9005 bulbs. Higher wattage bulbs are generally not recommended as the Miata harnesses have been known to melt under the higher power demands.
There are several manufacturers that make dual tip mufflers for the Miata. Turbocharged Miatas may benefit from a larger 2.5" or 3" exhaust. Please see related links below for Miata-specific vendors.