What is the actual error message? Over boost P0235 or P0234 sensor voltage?
Reason I'm asking is that I've had boost issues in the past and it turned out to be the multi electrical connection to the r.h.s. of the rad. I imagine that when your engine was replaced this connector was separated.
On mine when this happened, on running the actuator tests in fiatecuscan, the turbo actuator appeared to function correctly. However engine still went into limp mode with one of the error codes above when giving it some stick. Eventually,,,, time passes.........I found a 50ohm resistance (on the solenoid circuit) across that previously mentioned electrical connector. Ended up having to bypass the offending wire and that cured the problem. Check that first before laying out £70 + for a new solenoid.
Need the error code though, I take it you have a diag program?