Hi Johno,
For the wheel/tyre combo, if you change the wheel size of your donor, the tyre dimensions will change too (obviously
). You have "equivalences" of tyre sizes (ie if you want to fit 15'' wheel instead of the original 14''), but of course, they are not "exact". It is just a question of how many % difference with the circumference of the original wheels you are ready to accept. I think between 0 and 3%, it is acceptable for your speedo/mileage counter to be still "correct".
I used a French website to calculate mine, but I found this one in English. It is about Miata, so it is even better for you !
http://www.miata.net/garage/tirecalc.html
If, as Ben says, rear arches are 11'' wide, you can with no problem use a 7x15 wheels, same ET (offset) as the MX5 wheels in 14'', and a 195/50-15 tyre (-0.3% difference with the circumference of the 185/60-14)
Now, if you want your wheel to be as close to the edge of the arch as possible, you will have to recalculate the ET you need. But bare in mind that not all ET are available for a given size/centre bore (register)/PCD, so, I would stick to the MX5's ET.
Hope this will help