you can use the 200B's L20 engine easier than the 180B's L18 as the sump configuration matches the 1200. The other L series engine with the right sump is the Stanza L16, not the 1600's (510). Only real difficulty with the L18 is the dipstick location. You can bolt on the correct oil pickup and sump from the 200B/Stanza, but you have to modify something to get the dipstick in the right place - eg drill block or put a tube into the side of the sump.

79 onwards 200Bs struts and brakes (244mm Girlock caliper - same as Stanza) are a good upgrade for a road car. But I can't remember what spring size they have...