Hi Adam
You may want to take a look at this web site; this builder in the USA is planning to fit a Mazda Rotary engine!
http://www.trv7.com/ I actually work for Mazda UK, so I can obtain technical information regarding these engines if you need it.
However if you do want to go down the long and protracted route of fitting a rotary engine;
do not use a second hand engine or an engine that has supposedly been overhauled ! No Mazda dealer in the UK or the rest of Europe would contemplate a major overhaul.... we don't even allow repairs during the warranty period of a new car.
If any problems exist we simply replace the motor.
The good news about using a new rotary engine is the cost; which has been drastically reduced in the past year.
Hope this helps
Regards
Mark