i have a 1995 miata 1.8 that i just rebuilt and put about $1500 into the motor (flowbenched, shaved head, port polished, balanced, pistons, rings, ect.)
i looked into the swap myself and talked to a few guys who used the 323 as rallyx cars for about 7 years.. they have built and modified the 323 and motors more than alot of people in the us.
the hardest part in the miata swap is the intake.... which if the motor is put in a front wheel drive car works, in the miata however it hits the firewall and is more or less mounted backwards. this wouldn't be such a big deal but that is really the only way to run the charge pipping (unless you go really custom)
anyways good luck and let me know how it comes along
Grayson Aka shopmonkey
95' miata Type R
84' Vw rabbit GTI
70' datsun 240z