This happened last year to me as well.Hi,
The original engine (on 108k) is the one with blown piston, this had a new timing chain kit fitted at 104k, I have the invoice to prove it etc.
The replacement engine is on 41k, I will check the wear with the timing chain tool, just to be sure. Will also be replacing crank shaft seals etc before fitting the engine.
I have a guy that can pull the map off the ECU and let me know if its stock, or a bad map. I don't know if its been mapped or not, I only just bought it.
The engine from the other car however (the 41k), i have checked the MOT history and it's got a glowing report.
I had timing kit, walnut blast decoked & auxiliary belt done.
I went back a few weeks later (as they didn't have time to fit me in) and they checked the work they've done and said it was OK.
I got the car back smoking out of the exhaust and no boost. The MINI specialist tried to say it was the turbo, but I knew the turbo was working when I took it in. Turns out they removed (intentional or not) the turbo hose underneath the car when I had a replacement turbo installed at another garage.