As far as legality goes IIRC most of the mods are completely illegal. The 6′ tail pipes for instance. But for the annual licence check the car needs to be in standard (or close to) condition, only at that point, afterwards you can pretty much do as you please. The shoddy work being referred to is part of the look, it’s their version of the rat look.
But yeah, proper Boso are more of an outlaw motorbike outfit, pretty imperial right wing and prone to just doing as they please and to hell with consequences. Original Midnight Club street races allegedly got shut down after some Bosos challenged a racer and the ensuing race ended with fatalities.