Will 13" wheels fit over std calipers and discs?

Yes if it's an early 1.6 with the smaller brakes

No if it's a mk2.5 sport with the larger brakes

Maybe if it's got standard 1.8 brakes. It will be very close but some 13" do fit.

Alley Cats for instance do with only a couple of mm clearance. I would imagine prorace 1.2 and CXR's would as they seem to clear better than anything else. All with the correct offset of course.

Just for info

Rally design have started doing an MX5 kit using wilwood powerlight and have midilite in the pipeline.

Flyin Miata in the states do several kits inc a little disc option.

http://www.flyinmiata.com/index.php?deptid=5688&parentid=0&stocknumber=14-16175%2010IN

http://www.flyinmiata.com/index.php?deptid=5688&parentid=0&stocknumber=14-16150%20%2010%20INCH

Its a 94 1.8 ...