I wonder if the mower shop might have made an error (I don't say they have, but it could be worth checking). The B&S engines normally have a feature they call EzySpin, which holds the inlet valve open 0.010" during the first half of the compression stroke. Because of that feature it is important to check the tappet clearance with the engine on top dead center on the firing stroke. You might check that valve clearance yourself, in case they set it with the camshaft in the wrong position. If they set the clearance with the camshaft on the EzySpin part of the inlet valve cycle, they will have disabled the EzySpin feature and made the engine rather difficult to pull over.
The other obvious possibility is the one you suggested: the engine could be too tight. However you said it spins well with the spark plug removed, so that sounds less likely.
