2000 Opel Corsa Engine problems please help.

Engine Mechanical problem
2000 Opel Corsa Front Wheel Drive Manual 125000 miles

Last month I took my car for a major service at GM in Carletonville.
After a few hour they phoned me and told me that there's some serious damage on my engine especially on the crankshaft. They then ordered a reconed motor to replace the faulty one.
After they made the oil change after 1000km, I picked up a few problems with the car.
First, when I remove my foot from the accelerator while driving the car starts to shake and it feels like something pulling me back.
Secondly, when I reached a traffic light, my car was idling above 20000rpm.
After a while of driving, my car just stalled while driving.

Please let me know what the problem might be and if you require more information.