...be right hand drive.

See if you find the manual. It usually is in the language of the country the car was delivered.

I bet it is in german ;-)

Most parts not related to the engine are interchangeble with the other W 123 models.

The M 123 engine is a pretty strange thing. It was basically developed for the then new W 123. Therefor pretty none of the parts of previous engines fit. The M 123 was basically designed to sound beautyfull. It is a pretty hefty gas guzzler, when the carburators are not tuned right you will see some pretty impressive low gas mileage. On the other hand the M 123 never breaks. Never.

Send me the VIN to eberhard@ikonengold.de and the list of parts you need, I might be able to check on the EPC for the part numbers

With those part numbers you should be able to order the parts through the normal dealer network. After all, it is a Mercedes and having parts available worldwide is part of their strategy.

Kind regards
Eberhard