I Have never seen them jump time on the chain. usually they get loose and the chain wears on the cover, cuts into the coolant passage and leaks coolant in w/ oil. there is 2 styles of covers too. i have had to rebuild atleast 10 of these at work.
hot trick is to put a 20r head on a 22 block. old 20r’s are found in 70’s early 80’s celica/supra/trucks. you can use a carb engine, just swap your efi shit onto it.