there's info on this in the "engines into 1200" section of this website.
however, it doesn't seem to cover all of it.
1) you'll need a 180B crossmember, engine mounts and master cylinder.
2) try and get the control arms and swaybar too.
3) you'll want to upgrade to stanza struts and discs with 200B calipers while you're at it. the springs, from memory, need lowering by 43mm exactly to get them back to std ride height. if you want to go lower, short stroke KONIs are about $400odd a pair for the front. try and adapt the 180B swaybar top the stanza struts. if it doesn't fit, use the 120Y one. i can't remember if i bought it and it did fit, or if it didn't....
4) use the 120Y castor arms. they're shorter than 1200 ones and should fit perfectly.
5) gearbox will be fine in the tunnel. you MAY need a custom gearbox adaptor plate, depending on what gearbox you use.
6) tailshaft will need to have the front yoke changed.
7) be nice with the clutch, or you'll snap the diff like a toothpick.
this is the EASIEST and honestly the CHEAPEST way to fit an L series engine into a 120Y or 1200. not sure on bonnet clearance though with the L20B, but i know the L16/L18 are an easy fit with the 180B crossmember.
i have seen people use a stanza crossmember, but i haven't done it personally so i won't comment on that.
*EDIT* yes it all should bolt up perfectly, and your front track will be increased by about 15mm on each side. no neg camber unfortunately...