Wot up!

A friend recently bought an omega 2.2i sport. However the engine is kanckered on it and he considering an engine he found a private dude was selling locally. The new engine has ran around 25,000 miles and has service history too. It came out of a MV6 omega and is a 2.6 V6.

2 questions - Will this engine go straight into wot was previously a 2.2 without any problems?

and 2nd of all, answer to which im pretty sure i know, but:

The mileage in the car shows 126,000 odd. Is there anyway he can replace this reading with the correct mileage of the new engine?