Well there is not a hole lot you can do for little money to get a NA RX7 to go fast.
some things that i woudl suggest you to do are weight reduction types of things.
airpump and bracket=8lbs
IAC valve with silinoids=1.5lbs
A/C=58lbs (including all lines and such)
ect ect
You may also want to look at pickin up a set of aftermarket rims. Dont have to be the best thing out there but if there lighter than the ones you have on there then you will put that power to the ground much better.
As for performance upgrades, like you said you know of intake and exhaust.
But you want a full cold air intake with cold air box isolated from the engine bay. This is going to gain you some noticable power.
Next you want to work on a full block bacl exhaust. 2.5" piping is the best as you want to retain some backpressure in the system for thisng to work the best.
Next i usualy go with ignition. There is no emediat gain to be had with ignition but with this you insure that the fual and air mixture is getting burn fully. If your existing ignition is aged then this is a good thing to do.
After that i go to the drivetrain of the car. Start off with a good clutch. ACT, Ceneterforce, Bonze, Clutchmaster all make a good clutch for the car. The have a look at the flywheel. The less rotating mass, the faster you get the revs up and the sooner you are making usable power. I know you said you dont want to change the rear end. But often this is a cheap way to get more perfoamance out of the car. Find a used LSD and rebuild it. (if you dont alreayd have one)
This is all i can really think of right now. Othere than taring down the motor and doig some port work and such. But you do all the above and you will notice a good gain in performance of the car.