5hp Briggs flathead build

Okay everyone so i was given a 5hp briggs flathead by my uncle. it had a broken crank, so its been sitting on a shelf for the past 6 years or so. Recently, while i was working in the shop at school, i came across an identical engine. so i stole the crank from it, pulled mine off the shelf, and slammed the new crank into the engine. bolted everything up, and without the cover on, turned it over a few times to make sure everything was going to be alright. it turned over easily, good compression, goes through all the strokes without hitting anything. so i put the crank cover back on. All of a sudden it is very stiff to turn over. if i loosen the top bolts on the cover, it turns freely. Anybody have any idea what is causing this?
BTW the reason it needed a new crank is because it used to be a pump motor, with no hole drilled in the crank, and someone drilled a hole and put it on a gokart, but their hole was off center and it snapped the crank off.
Any help is appreciated, as i would like to build this engine and put it on my gokart.

Were there shims between the crank and the crankcase cover? Maybe there's one too many on there now.

no, i dont believe so. it is starting to loosen up just a little bit, since i let it sit these past few hours. i guess maybe the new crank has to get broken into the engine? i guess i should probably keep an eye on the oil for the first hours of running it