Virtually everything on 2a and 3 are interchangeable even if only as complete assemblies. There are a few minor problems - for example the windscreens are not interchangeable because the hinges are different - but the glass is interchangeable, or you could modify the bulkhead to take S3 hinges.
Transfer cases are fully interchangeable, gearboxes are interchangeable except the clutch piping is different, and so on. Except for the bulkhead, almost all the body is either identical or interchangeable panel by panel.  Front and rear axles are the same or interchangeable, except that the 109 went to the salisbury rear in late production. Note that there was a gradual change from British to Unified threads throughout 2a production, with metric threads introduced in late S3 production. 
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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