I can't really get any more specific without diving back into the game, but every so often when I pick a modded car to work on, it loads into my shop without an engine, so I just mark the job as uncompleted and send it on its merry way. There's no way to work on the engine of a car when the engine doesn't exist. I'll have more specifics once I dive back in and find some examples. I'm pretty sure the BMW i8 is one of them, though.