Here are a couple of places to look:

http://www.wankel.net/~krwright/cars/rx7/manuals.html

At this site you can down load factory service manuals. This will give you part numbers that can be used for comaprisons.

www.mazdatrix.com

There are some rotor and flywheel weight listings at this site.

Earlier rotors are heavier but have lighter flywheels. Later rotors are lighter but have heavier flywheels. Counter balance weights are not the same between early (light flywheel) and late (heavy flywheels) 12A's. Those that have mixed and matched without rebalancing the assy have reported engine virbrations.

I have no idea which housing or iron is better. I am certain that a few calls to engine builders could shed some light on your questions.

A great place to look for vendors is www.rx7club.com

http://www.ludwigmotorsports.com/ - this guy is an IT racer