I need a replacement engine for my 420. The car has about 90000 miles, the original engine is dead and cannot be repaired.

How do I find a good "used" engine with low millage and how can I make sure that the milage declared by the seller is accurate?

I have been told also that I could install a 560 engine in replacement of the 420. Can I do that?

Any rebuilt engine out there?

thanks in advance for any tips

Christian
PS: english is not my first language, sorry for the bad spelling