motor no longer turning.

I know this is a common thing with the outcome being either the blade jammed or pully snagged, but im certian this isnt the problem,



I had two mower both the same, on had a scrap deck with good engine, the other had a good deck with blown engine,



So I have swapped the two engines I have a fair bit of experience with this kind of job and never had a fault,



Basically now when I pull the chord it wont move at all, and also the blade is really stiff to turn, I have tried both with and withouth the plug in and still the same,



I cant get my head around it, I know that engine was perfect before the swap,