I finally got around to swapping all the 128 specific bits and pieces from the frozen 1300cc engine to the 1500cc mill I sourced from an X1/9.
The main differences relate to the fact that the SOHC engine is mounted at a different angle in an X1/9 versus the FWD 128.
The X1/9 had a cast aluminum oil pan. The 128 appears to have a stamped steel pan which is shallower than the X1/9 variant.
As a result the pick-up foot on the oil pump had to be swapped over. Also, the breather vent oil return tube is also 128 specific (it is shorter). The only other item other than the oil pan itself is the dipstick. The X1/9 block has an aluminum dipstick holder threaded into the block along with a much longer dipstick. I think I am going to remove the holder (as the 128 came) and use the shorter 128 dip stick.