probaly worth a shot, trying a different coil, i got a msd one( was around 200) and it free revs fine but as soon as its on the dyno breaks down and shit after 5000, a mate of ours had the same problem with a datto
bore out to 60th, shave head until 12:1 compression, bigger cam, extractors, weber if your allowed, port and polish head, bigger valves and springs, fully balance motor, deck block and yeaaah
did u check the timing? if advanced to much it can blow them. i don't one last week like that. also make sure when u tension you don't use any extensions.
tbh i have never heard of anyone running them in a ke road car.. i run them in my speedway car but we move the radiators into the rear.. i just tried a spare one into a corlla we have and fits fine? just need to remove the fan at a guess and put in electric
About the jets on the 4k carbie, it would be best to go and buy some jet drill bits.. only up your secondary jet because if you up your primary it can flood the car easy.. just drill it out a tiny bit each time, take it for a drive then check the plugs (you wont your plugs to be a tiny bit darker then a a creamy colour), and just keep doing that until they look good.