Backup your database prior to going through the process you describe.
I will suggest using the SMF
Large Upgrade package to fix your forum. After doing that prior to installing mods anew, it would be best to remove all hooks using this tool:
Hooks Removal Tool.
After the above process, you can then install all your mods again including Simple Portal. Any SP data including blocks and pages should remain with the re-installaion because the SMF Large Upgrade package will not alter/drop those tables in your database.
Backing up your database prior to the above process ensures that you have a copy of your forum data intact in case something goes awry. If you also wish to make copies of files to be safe imo the most important ones to backup are the attachments and avatars directories. Although the SMF Large Upgrade package will not alter those directories either. All it does is put your main SMF files to default and render all mods uninstalled.
Here is the link to the wiki for using the upgrade package:
http://wiki.simplemachines.org/smf/UpgradingDon't let the name of the package fool you.
It is for upgrading from older SMF versions and/or fixing forums that are messed up.
Regards.