Welcome to Texas Spec7
Home of the Texas SRX7 Road Race Series
That is not a spec7 car and about the only regional class that thing could have ever competed in was autocross. That was never an on track wheel to wheel race car. Got to love CL. http://dallas.craigslist.org/dal/cto/2905734150.html
Rules require '81-85 12a only.
There were a pile of 12a's and a 13b for sale on CL recently for $500 or so, about 4 engines. They appear to be gone now though. Watch CL and eBay. Do NOT buy from the guy in Rockwall, trust me.
An ITA RX-7 motor should swap over, at least as far as the shortblock is concerned. As far as legal for PTF? Highly unlikely. A SRX7 barely makes PTF and ITA allows more modifications than SRX7. I would not buy an ITA RX-7 just to have the motor. If you can't find a 12a or the parts you need locally, buying on forums is not that big deal. I just shipped a shortblock from Dallas to IN for $130. As long as your engine is not completely shot, it should be about $2k to have it rebuilt parts/labor/shipping and good enough to beat anyone here.