Just create a new tuning sheet (add page, not copy).
The option to do this is directly next to the name of the page.
This will remove all parts that can be removed, so not including weight reduction and the engine upgrades like bore up, and will restore the default settings to every tuning parameter like suspension and so on.
Adding to that: purchasing a new engine will remove the upgrades that are still left (like TumeK5 already said).
