in reality the money is better spent the stock motor. i have considered teh 13b swap many a times but its too much money. for starters you need a custom oilpan, even tho i heard the GXL Rx7 oilpan installed backwards will help a lot in clearance. then you need custom motormounts because they dont line up at all. and if you really want to get technical it doesnt sit anywhere near where the stock miata motor sits so a bunch of custom brackets need to be made. then you need a custom tranny mount to hold the tranny since the rx7 bolts up (with a different bell housing) but is much heavier than standard miata tranny.then you need a custom driveshaft because the RX7 Tranny is longer than the miata transmission. then, since you have to take out the miata's stock PPF bar ( bar that holds the tranny aligned with the Differential) you need to find a custom diff mounts or spend the big bucks and get that whole TII conversion kit thats like 1500 bucks. all in all its need way more than just " a little bit of work" to make it work properly. this is deffinately a job that is not ment for the weak of heart. takes a lot of mechanical experience to pull something like this off. so if you feel you can do it then go ahead it will be fun so thats for sure.
and if you plan to do this just to be different, just know its been done before and often so its getting played out in some parts of the country....i still dont mind and would love to do it, but i plan on playing with my stock motor for a while.
hope that info helped some, and if you got any more questions ask and you will be answered