A12 engine: 87 kg
CA18DET: 128 kg
plus you have to fit a heaver diff, more unsprung weight in the back doesn't help handling.

How does this affect a 1200 in reality?

I can't speak from personal experience, but a bloke I know used to have a 1200 coupe racecar. He started out with a wild A12. He build adjustable suspention ect for it. He said that despite getting owned on the straights, he dominated everything around the bends. Then the motor went bang.
Swapped in a ca18de. The car was slower than with the a12 around our local racetrack.

Swapped in a CA18det,
Car went like a cut cat. But, it was no longer the weapon it used to be in the corners.

in a properly set up race car i'd say you would notice the difference.

in a street car...i doubt you would notice it
dont think many people keep the exact same suspention and wheels after a conversion

Uprating springs and some good shocks and you would get it handling better than ever before .
Cheers
Bob

If you put the good shocks and springs in the A series car it would out handle the heavier CA powered car.

I have run my car with an A14 and also a CA18DET.

While the car is a lot faster these days with the CA it no longer has the crisp turn in and mid corner adjustability that it did with the lighter motor. I tend to drive it more on the throttle than simply via steering input nowadays (I hope that makes sense).

I wouldn't say it handles worse these days but it isn't as much "fun". Fortunately I have also owned Bert's 1000 coupe for the last couple of years for that.