Way back in high school, I got a great deal on an XK 120 Jaguar..Straight six with chain driven dual overhead cams..The rich kid who's mechanic told him the engine was shot didn't check the cam chain, which had a broken tensioner..about $50 bucks as I recall..that allowed the exhaust cam to skip one tooth over and made the motor not run real well.
  Then there is the Porsche 928 which is an "interference motor" for sure... I know, having broken a timing belt at Road Amerca race track and trailering home to Portland, Or. with 29 of 32 valves bent by clashing with the pistons..4 cams and an idiotic drive belt  system for those cams..
  Ducatti had a desmodromic valve train..One tower shaft driven gear between the cylinders with some kinda linkage to both open and close each valve...no springs..
  And the two stroke motorcycles...no valves at all, just ports that are opened and closed as the pistons go up and down..
  The Rabbit motor in my van has  a dorky little toothed belt under a snap on cover..Looks like a fan belt, or maybe the power steering belt off my big Porsche motor..tiny little deal and easy to change.
  Don Hanson