I do not know what engine was originally installed. I also think it's safe to assume that the miles on the current engine are much less than the miles shown on the odometer.
I do not have photos from what it looked like back in 1978 or prior to my ownership. It was, and currently is, titled and registered in Massachusetts as a 1978 Caterham Super 7.
I don't know if you can separate the two and slide them apart enough while still in the car to change the clutch. Removing the engine and transmission isn't daunting and no special tools are required other than a hoist preferably with a load leveler,