If you are looking for a cheaper alternative to a V8 swap in your Miata you may want to consider a Honda J-series V6. Mini Tec sells a JV6 swap kit for $3,500 that will allow you to swap any 1999-2007 Honda J engine to any NA or NB Miata (1990-2005). Although the 3.2 L from a 2003 Acura CL Type-S with a manual transmission is probably the best option since you can use the stock ECU. The beauty of this kit is only needing to replace the engine. Mini Tec supplies you with everything else you need in the kit and the transmision, driveshaft, diff, radiator, and everything else can be kept stock.
Source: Mini Tec