Transplanting Japanese engines into BMWs or Mercedeses are actually quite normal since the 80s. During those days if a Mercedes carburettor, a solex I think, costs about 2Ks plus. It can't be serviced and needs to be changed so the foreman will recommend transplanting an Isuzu JJ engine. It will save cost in the long run but not perfect. The performance will never be smooth. Likewise, when I driving a BMW 320 (80s model) I have the same problems. Maintainence starts to get problematic and expensive after the car is over 7 or 8 years old. The foremen did recommended the same thing but as a die hard BMW fan, I resisted.

If you ask any Cameronians, Japanese engines are more hardy than continental makes. Almost all Land Rovers there are transplanted with Toyota's diesel engines. They say it is more reliable than the original Land Rover's engines which leaks a lot. They even modified power steering system into the old 109s Land Rovers. Ultimately the main factor is cost and the abundance of spare parts. even in today's time, any continental parts from half cuts are still more expensive than Japanese.

Thinking of it, if you transplant just the engine only as TS said, you only maintain the engine itself cheaply. With proper and strict maintainence schedule ( which is very important for conti cars), the engine rarely gives problems. It is the non-compliance to these schedule maintainence that makes them running smoothly hard. Back to the subject, other than the engine parts, you still have to maintain the bodyworks, the electrics, the brake assembly, the steering assembly, the gearbox, the seatings, the air conditioning systems, the bearing systems on the wheels etc. which also contributes to the high cost of the maintainence. Think about it, what do you really maintain in an engine block. Regularly the lubricant, oil filter, air filter and plugs. Other than that, the distributor system, throttle and fuel injection system which rarely gives problem. The most headache part of old continental cars is the cooling system but with proper regular flushing of the radiators and hoses and scheduled water pump change, the problems could be reduced.

In my opinion, I wouldn't go for Japanese engines as I don't see much difference in maintainence cost savings. Just think back about your track record of going to the mechanics. what did you repaired the last time. Was it the throttle? The fuel injection system? The crankshaft? The head assembly? Or the brakes, the exhaust, the air cond, the air cond. If maintainence is really getting too expensive for you, might as well sell the car and get a Japanese model afterall the mechanics will treat repairing your car as a Bimmer rather than a Toyota.

I had a friend who transplanted a Cefiro engine into a Mercedes 123 but problems still persists and finally he sold off the Merc but at 10K less than the actual value because of the engine transplant.

Just my thoughts.