Other engines in an RX-8 (sorry for all the questions)
 

Any suggestions on other engine that can be relatively easily installed in an RX-8 to alleviate reliability issues?

Not for less than the price of a rebuilt stock engine, unfortunately. If you have 10k or more to play around, Chevy LS V8s are a well explored option.

Then you may as well just buy an early 2000s Mustang. They are dirt cheap.