These things should be timed. Beating the Executives and Other Criminals single-player port mod in terms of turnaround time, the new custom rides introduced in the newest GTA Online DLC, Custom Classics, have now been added as spawnable vehicles in single player thanks to a mod.
GTA Online has been seeing fairly frequent free DLC updates ever since it was first launched soon after GTA V back in 2013. However, unfortunately the game's solo portion has remained unchanged and thus un-updated ever since launch. Aside from bug fixes, the story mode of the game never saw official new content.
It wasn't until the release of the final port - the PC version - that this would change, at least partially. GTA V on PC supported the use of third-party mods, allowing players to expand their single player experience for the first time. Once modding caught on, it wasn't long before DLC content was being ported over for solo players to enjoy too.
Of course, GTA being what it is, the new vehicles were the main attraction. Many of the new game mechanics introduced in Online DLC only works as multiplayer, thus porting it to SP is impossible. DLC cars started appearing in port mods all over.
While the other DLCs also added their share of vehicles, the Lowriders update was the one most focused on vehicles. A particularly industrious modder managed to port all of Benny's Original Motorworks over to single player, including all of the upgrades.
Since then, Benny's has seen two updates in GTA Online, with the most recent one adding three new fully customizable rides to the game. By "recent" we mean "yesterday", so someone getting around to modding these into single player so soon is quite the achievement.
GTA V modders _CP_, WildBrick, and robi29 have gotten around to creating a version of the game file "mplowrider2", which contains the new rides that allows these vehicles to be spawned in single player using ScriptHookV's ASILoader function. Note that the use of this or any other mod while playing GTA Online is strictly prohibited and will result in an immediate account ban.
Apparently the mod also contains the new rides which will be added to Benny's in Online later on. The trailer for Custom Classics did tease more lowriders coming soon, and this mod introduces more than just the custom Slamvan, custom Virgo and the Donk.
Which GTA Online feature would you like to see appear in single player?