The myth continues: the 3rd gen RX-7 had an unreliable engine.
The 13B TT was a very reliable engine. It was just not overengineered, particularly in the cooling department. This means it worked well and reliably when run as it was designed. However, too many people tried to alter the design without adding the capacity for those alterations, and the car broke.
I think most rotary enthusiasts would be very offended at the idea of taking a Mazda RX-7 and putting a piston engine in it. I am tempted to post this on rx7club.com and see the response.
If you want a V8 Mazda, take a Miata and put a Ford 302 in it. This is a popular mod which the enthusiasts insisted remain possible with the 2nd generation.