1. There are many things you can do to the engine, intake, exhaust, compression, timing, cams, etc, etc, etc...
2. A stock SX Corolla strut brace should fit and would be the better choice due to a three point contact - strut, firewall, strut...
3. KYB or Boge Gas Shocks are always a good choice for struts...
4. Not too sure but I think the blacktops all came as a 6 speed gearbox, but were non LSD... The only LSD boxes were in the 5 speeds and they are getting hard to come by it seems...
5. Custom made to your specification is always a good way to go when it comes to manifolds, but you're better off doing your research into 20V specialists and seeing what they recommend... Try www.twincam.org as they are a more so AE92 based forum dedicated to the 4AGE engine series...