Vehicle modification options in garage

Hey guys, my question concerns the ability to modify vehicle performance using the menu screen in the garage once you go on duty. Specifically, while I know that vehicles under the model categories of police1 - police4, the sheriffs 1 & 2, fbi 1 & 2, etc. are amenable to having their performance tweaked but that the "policeold" both 1 and 2 are not. Is there a way to somehow enable these adjustment like engine type, turbo options, bulletproof armor, etc., to apply to vehicle models that you've used to replace the original policeold1 or 2? Any insight or help would be greatly appreciated. Thank you for reading.