Defo Cut your losses
and get a new mechanic

A good mechanic will give you a reason why the engine has to be changed, Pre ignition is a unrequested combustion which will be fuel and heat realted timing, It can be easy to diagnose with trial and error testing. Did he do a data logging session using vagcom?
Can you shed some more light on the fault?
Do you get a fault code?
Symptons?
Conditions?
All the time or every now and then?