With those terrible Kadrons, Ryan has been motoring all over northern California pretty happily for the past few years - even on some roads that were paved.
No popping, no misfires, no stumbles, no cutting out, no pushing the car back home.
I suggest that it's not a general failing of the Kadrons that has made for a pretty sudden change in how the motor's running. I'd get these ones back to running like they were first and then decide if it's time to upgrade.