Got a buddy that's turbo-ing his miata now. He just picked up a GT2860R and he's lookin for 250 whp on low/med boost.

A 20b will fit, there's a video floating around of one at an Auto X track.

But unless you've got 10-15k to blow you'd be much better off droping in a 13BT. You can easily make 300+ hp and Mazsport has a conversion kit.