While many players use edited saves in online free roam without issues, you should avoid using "impossible" stats or mods that give you an unfair advantage in competitive PvP races. This is where you risk being reported by other players. Conclusion
Open the save editor tool and load your original save file. Nfs Heat Save Editor
Using a save editor for your solo campaign is virtually 100% safe. While many players use edited saves in online
Before launching the game, temporarily disable Origin/EA Desktop or Steam cloud saves. If you don't, the launcher might overwrite your modified save with an older version from the cloud. The Engine Swap & Ultimate+ Shortcut Using a save editor for your solo campaign
Boost your solo crew level to unlock the Ferrari FXX-K Evo. How to Use the Save Editor Safely
Similarly, you can use the editor to see which engine swaps provide the best theoretical stats before you spend your hard-earned (or edited) Bank in-game. Will You Get Banned?