Miata One is a company in Eagle, Idaho that makes a swap kit to install a naturally aspirated Subaru EJ251 flat-four into a Mazda Miata/MX-5. The swap kit is made for 1989-1997 NA and 1999-2005 NB models using a Subaru EJ251 phase 2 block with an EJ251 phase 1 dual port heads and Subaru five-speed manual. The phase 1 heads are used to give the most amount of clearance between the frame rails. The swap is not a bolt-in process and requires the subframe and crossmember to be notched. The kit costs $925 and includes motor mount adapters, bracket to connect Subaru transmission to PPF (frame), shifter mount and isolator bushing, alternator mount and adjusting bracket, fasteners, and installation manual.
Update 7-12-2019: Website/Company no longer active
Source: Miata One (website no longer active) via Piotr