I see no reason why either generation of KA24 (FWD) wouldn't fit in the u12. I did the swap with a 1st gen DE engine. Driveshafts are the same between the U13/U12 (I'm 99% sure) so you shouldnt have any problem if you use the U13 manual ones, no idea if they are the same as 2000 model though... you'll just have to make sure they match the box if its LSD/non LSD.
Engine mount fixing points all line up (may need U12 brackets). Plus you'll need to modify the radiator hoses, but I think the DE radiator is a bit bigger anyway so maybe swap that over too. Exhausts are different so you're best to get some headers done. Thats it from memory.
The hardest part is rewiring.
Hope that helps