The pics in these websites might give you an indication of what it takes to drop a 6 cyl engine into a 4 cyl 201 engine bay. The Diesels shown (OM603 and OM606) are about as long as an M103.
http://www.teamrwd.com/dieselboost/300_24.php
http://www.teamrwd.com/dieselboost/250.php
Sixto
95 S420
87 300SDL