The handful of 245s I checked recommend at least an 8" wheel.
Assuming you don't want to mess up your speedo, neither is a great choice.
225/40-18 is closest to your stock circumference, and probably the best size for a 7.5" rim.
There is a great tire size calculator here:
http://www.miata.net/garage/tirecalc.html
You would want to be within 1%, and could live with 2%.
Sorry, no Azenis Sports in 18 at all...
What are you looking for in a tire, and I can help you narrow it down.