The Japanese versions of the DB V-12 engines were really quite re-engineered in detail. The Fuel injection system is listed as not being a copy of the Bosch PZ12 type, but a locally designed and built type. This may have been a choice by the Japanese, as I cannot imagine Bosch refusing an RLM directive.
Rolls-Royce built Merlin, Packard built Merlin and V-1650 engines did have differences, sometimes quite major. The Magneto's and accessories were American specification but there were some major differences. The supercharger drive was a different type of mechanism completely, the Packard engines always had a Bendix Stromberg pressure carburettor (and very late with Simmonds FI) that was immune to negative-G, the early Packard Merlins had their own design of detachable heads (changing later to the RR type), propeller shafts had splines etc for American props and reduction gears could have slight ratio differences. Beyond that, the modification states of the Packard engines could lag the UK production.
That said, the majority of parts in the same build-state engines were identical and so, interchangable.
Eng