Background: about two years ago my mother's mini started to over heat and lose coolant, there was hole in the radiator and it was filled, the problem went away, the mini ran extremely well for the past two years - we were told this filler was not a permanent fix.

This has started again, boiling coolant in the bottle, coolant on both front rims, brown thick layer. We brought it to our mechaic where we were told the head had a crack in it, our mechanic replaced the head with a new one two days ago (21/04/19), hoping this would solve it.

It did not today (23/04/19) the coolant bottle was boiling when the car was turned off, no fan was running and the thermostat needle had not moved yet.
Our mechanic told us the engine is bust, I am confused by this extreme statement, the car was driven 130KM after the head gasket was changed, speeds of 120km an hour steady. No problems at all. The engine has never stalled never given up, it is a regularly serviced car with 130k on the clock.

I believe it is the radiator come back to bite, but the mechaic thinks it's the engine? How can it be the engine when the car is driving extremely smoothly, never misfiring or skipping a beat.

Any advise?

I am thinking of replacing the radiator and water pump to eliminate this before spending 2k on an engine....

My mechanic has been dealing with our cars for 10+ years, I do not doubt what he says, however I am sceptical if he looked at the coolant system and radiator due to saying it's the engine even though the car seems perfect..... He has helped us out on his own goodwill many a time, so I do not believe any foul play is about...

Cheers
Robert