why dun u install a heatshield for your mushroom open pod filter... that should help with the heat problem..

This one...

Exhaust manifold... looks like a crack in the exhaust manifold at the O2 sensor there (pic3).

You can try ask if the shop can 1) weld back... 2) find halfcut parts/other old manifold from same model... 3) replace new one (dunno if still got stock).... 4) replace with custom made manifold (the most expensive option)

I would recommend option 2.... if you can find the part, maybe get it for a few ringgit... normally when the owner of the same model change to aftermarket custom made exhaust manifold.. the exhaust shop just throw away as scrap metal. Half cut maybe more expensive a bit than getting scrap metal part. But they work the same.

the regulations is on noise, not on size... put as big a pipe as you want.

change MIVEC engine and then the exhaust to match the new engine for real world performance improvement.

If you just want psychological improvement because you spend $$$ and exhaust sound different, then just change exhaust.

Can't advice on the engine, since I never used Mitsu.

But extractor to me has negligible impact to noise.
That's the muffler and middle bullet job.