Let us also not forget that the motorcycle industry has also provided engines for some iconic cars. JAP (admittedly more an engine company, but definitely bike oriented), Matchless and Harley Davidson-by way of S&S-for Morgan. Yamaha, with their development work on the Toyota 2000 GT. Honda with the S600 and S800, whose engines were far more closely related to motorcycle tech than typical cars of the day, along with the N360/600 range with their unequivocal bike like engines.
Not to mention a thousand homebrew specials, of which the Bolster brothers' Bloody Mary is possibly the best known.
And, of course, the early days of F3, and what some might regard as a spiritual successor, the Legends class.
Cross fertilization in the other direction has been, shall we say, less successful, with such car engined oddities as the Munch Mammut (NSU, though, to be fair, quite strong bike influence in the engine design), the Amazonas 1600 (VW), and some fairly nasty one offs with Hillman Imp power. Oh, and this.