Decompile/Edit/Reprogram existing Inno Setup v.5.2.3 build

Hello - We have a installer built using Inno Setup v.5.2.3. I need it decompiled, and a new welcome screen added to capture user information (I have a html form that backends to our DB). I also need some minor re-programing done to the current build to change some default settings. Basically, we need a few tweaks and additions added to what we already have. This will all be put on a autorun CD to be distributed - so it has to look good! It's an offsite backup software. Don't hesitate to PM me so I can answer questions or explain more, I will try and answer ASAP - I am on US EST. I need to make sure the communication is good - and we all have a clear understanding of the outcome - some creative input would also be great - as i said, it has to look very professional!