if i were to put a CA18 into a 120Y what other mods would have to be done to the car i.e. brakes, diff ect.

brakes, clutch system, front struts, diff, gearbox, shock absorbers, trans tunnel, fuel lines, fuel system, whole lot of wiring.

It is not a 'bolt in', unlike a 180B engine and crossmember in a 120Y is.

To do it properly you will basically need to re-engineer the car. Most mechanical parts will need to be replaced or strengthened. There are several lists floating around the internet of what needs to be done. I'm not trying to put you off as a CA powered lightweight car is a lot of fun but don't start a project such as this thinking it's an easy bolt in job.
Regards Matty

i understand there is alot to do i just wanted to know what
thanks for your help