IT-7 isn't nationally recognized. It's actually a Regional class. Your region may, or may not support it by offering it as a distinct class. All RX-7 (first gen cars) must run ITA or lesser prep levels to be considered for IT-7 inclusion where it is offered.
The supps for any Regional will list the class if it is being offered, and should note the ITA prep level.
Specif answers to your questions:
ITA weight for the RX-7 is currently 2280. It IS possible to achieve that weight.
Any DOT tire that fits the IT requirements is fine.
Toyo makes another tire that replaces the RA1. Results are varied. Your tire choice will depend on your budget, your need to win, and the competitive level in your area. In my case, I run Hankooks.
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