Just gonna chime in my two cents with no experience with building or swapping an rx8. So all I'm doing here is putting out an option you can potentially explore. Just throwing around some ideas is all.
In my opinion, if you do want to swap an engine forego the 2JZ/1JZ swap as it would disrupt the weight distribution way too much. Some lighter options like a K-swap or something like an SR20DET or F20C would be better, I think K-Swap would be more viable as far as availability is concerned in Pakistan. You may not achieve the 400hp goal, but I think you can get away with 250-300hp with a slightly built motor. The weight distribution would still be ruined, however not by much if you would have gone with a JZ swap. A suspension upgrade with tuned coil-overs will definitely be needed to compensate for the not so perfect weight distribution, even then it may not be like a stock rx-8. But it's not like we have race tracks in Pakistan, so send it street light to street light and have some fun
P.S just get a rebuilt rotary, a tune, and enjoy some pop's and bangs for a good 50-70K km's