An engine from a 77 to mid 79 300D should fit no problem. An engine from mid 79 to 85 will probably fit but may need some modifications like adding the newer style glow plug relay from the later models and possibly the vacuum lines. Good Luck:thumbsup:
W123 engines will fit into a W115, but you'll need to change the position of the oil filter. I don't know whether a W123 turbo engine fits into a W115. There is less space in the engine bay because of the different front suspension.
If you do "upgrade" to a turbo, you'll need to change the differential. The turbo engine is best installed with its original transmission.
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