the diesel swap was pretty well bolt on, the 1.9TD did need some modifications here and there but overall an easy swap. the 1.6TD will be a bolt on for the most part. the casting on the block for the 1.9 needed some cutting and grinding to make it fit, starter is moved to the back of the motor so there were a few protrusions that needed to be cut and the same under the water pump, nothing too invasive though. it also depends on what alternator you are going to use and pulley system. the clutch cable mount had to be moved around the oil filter which does not need to be moved with the 1.6.
when i had the gas motor i did take it to 130km/hr and she did well, very loud but it had no difficulties. but they are not designed for speed and anything like that sustained will have the drive train and tranny start to come apart on you.she was very comfortable at 110km/hr with a gas motor, a lot slower with the diesel now till i change the gear ratios.