They really should have change the belt and pulleys.
If they have changed 2 pistons, then they have also changed the big end bearings, right? The reason I say that is because the bearings on these engines are notoriously soft, and
can be flattened after valve to piston contact. Failure do to so can is oil pressure loss and instantaneous major engine damge 5 miles or 5 months later.
I'm sorry to point that out, and hope it won't be an issue, but the more people who know the better.