There could be a very rough method for makinga nice adapter......As the old engine seems not to be a veryexpensive part, You might simply cut off the last few centimeters, (I think You might do this..as you cut down the citroen crankshaft....)and use this as adaptor for Your motor.The center is defined by the bearingsThis will fit perfect.
Yes. That's the plan.)
......I did not mean only the crankshaft,I thought to cut off the motorblock too!(cast iron is a good material for milling)