Wondering if I can put the 87-91 Mazda RX7 turbo engine n a 91 Mazda RX7 convertible with the stock auto tranny?

The original rotary is your best bet- trying to modify to a turbo will be WAY expensive- it is best done by an expert in RX7s-

Short answer is yes, but I don't know how much power the auto will take, a lot of the JDM motor sellers will sell you the motor & trans and most of the hardware to get the motor running, you'd still need the turbo rear end and drive shaft. The turbo motor should bolt to the na flexplate and converter idk if the trans is ecu controlled like the 3rd gens. Making it harder to swap. A lot of turbo verts are out there tho