The best similar size, with a good variety of well priced tyres available, would be 205/55 x 16, but i don't know if you have the clearance necessary on turns, might be a question best answered on the MX5 forums, obviously a change of size would need notifying to your insurer as a modification.

Alternative sizes, if allowed, are usually shown on the tyre pressure/fitment plate on the door shuts, and noted in the handbook.

At some 73mph compared to your size at 70mph, sppedo will probably be more accurate, as the vast majority of speedometers overread anyway, you could check yours with a standard satnav compared to the car speedo before purchasing anything at all.

Edited by gordonbennet on 18/10/2019 at 15:27

https://www.oponeo.co.uk/advice-and-tips/tyre-size-calculator/205-50-r15#alternativeTireTool

A very useful tool for checking to see if older cars with less common tyre sizes have compatible ones, but (OP) you'll need to check with Mazda to see what range of width of tyres can be fitted (I presume you don't want to change to another rim size) without inpinging on the brakes, suspension components and wheel arch, whilst staying within the legal +1.5% / -2% from the OEM tyre diameter/circumference.