Duh, it's a handling mod so he has to replace the lines of the Infernus by those. Follow these steps:
Go to your GTAVC data folder. (C:\Program Files\Rockstar Games\GTA Vice City\data)
Make a backup of your handling.cfg file (Copy and paste it somewhere else in case the mod mucks up)
Now open that file with Notepad.
Then go to Find and write INFERNUS. You should get results.
In the mod's readme or custom document, etc. copy that number line (Not the name).
Get back to your handing.cfg and erase the original number line of the Infernus.
Paste that one.
Before saving, go back to the folder and right click handling.cfg and then click Proprieties. Check if it's Read Only.
If it is, untick the box and click OK. Now save the document.
Fire up the game and enjoy. Just be sure it doesn't crash as many people paste the line right after the name with no spaces.
Enjoy