The threading is 20mm x 1.5. It is located underneath the intake manifold (if looking directly at the engine from the front of the car, it's on the left side of the engine). If your arms are not thin enough to reach under the manifold from the top, you can reach it by removing the wheel and accessing it from the fender.
Here are step by step instructions and how to changer miata starter : http://www.racecaddy.com/mx5/tech/starter-mx5.html enjoy
Here you can find the whole instructions how to change your timing belt on a Miata http://www.racecaddy.com/mx5/tech/timing-belt-mx5.html it has good images and it is well explained good luck
You say MX5, not Miata, so do you really mean 2006 or newer MX5? If you really mean 1990-2005 Miata then the water pump is behind the timing belt cover. It is not driven by the t-belt, but the t-belt must be removed to change the pump.
The battery is a Mt-MX5. This is a Interstate battery part#. fits 1990-2008 Mazda Miata. Go to interstatebattiers.com to find a dealer near you.
To purchase a second-hand Mazda MX5 Miata car, one should visit the local Mazda dealership. Alternatively, search through newspaper listings or ask a mechanic to find one.
Miata Starter is located on the passenger side of the engine at the bottom end of the engine block, slightly behind the oil filter. You can find images and info how to change it here: http://www.racecaddy.com/mx5/tech/starter-mx5.html
It is hidden behind a plastic shield underneath the car, just before the rear, on the passenger side tire.
As of 26 October 2015, the latest ND Mazda MX5 Miata only comes as a soft top roadster
10W30 or 5W30 will work just fine.
I found www.mx5parts.co.uk to be the best for all mx5/miata parts, good price and excelent customer service.
Many different engines can fit into the Miata. See the link below to Flyin' Miata, a company that does such engine repalcements.
There is none, except for Japanese versions if I recall.