Hi Simon - here's some info about a L18 that someone put in their 1200. I reckon there are better choices out there now but maybe this will help you make up your mind. This is from Martin
Pots Website at http://members.iinet.net.au/~mpot/DatsunMods/
... has heard of someone who installed an L18 engine into a 1200. Then entire job was done by an
apprentice mechanic. It was a huge job, and involved deeply scalloping the crossmember, engine mount mods, and reshaping the sump.
The engine runs custom extractors for clearance, with a 2.5" system and 3" tip. Induction is twin 45 side draught Webers, which give an incredible throaty roar under acceleration. The
head was extensively worked, with maximum porting, valve increases and a huge cam.
A factory EFI setup imported from Japan was tried, and found to be much smoother, but too restrictive above 6000rpm when used with the cam. The gearbox used is a reverse H Japanese race
box. The radiator is a custom 5 core with a big thermo fan. The diff is a shortened 1200 ute diff.
The engine produced in excess of 170 hp, and was incredibly quick!