Welcome to the site !!
If its locking up as your kicking the engine over , its going to be a timing issue , either you are off on the timing and the open valve is hitting the piston , or you let the chain drop when it was disassembled and the chain is bunched up on the bottom sprocket hitting the deflector that keeps the chain from falling off the bottom sprocket behind the flywheel , or the Tensioner is over extended and jamming the chain , I am hoping you removed the tensioner when pulling the gear , if you did not it will get caught and pop out , not only possibly damaging the tensioner , but allowing the chain to drop off the bottom sprocket and causing the lock up
You need to test to see if the chain is on correctly , so remove the spark plug , and remove the tensioner , then remove the sprocket from the cam , and hold the chain snug (you can use a screwdriver handle or something in the sprocket to allow it to spin , or just remove the sprocket and hold the chain with the handle , now , while holding the chain snug , turn the engine over by hand (or have someone help you) while holding tension on the chain , see if the engine will now turn more than what it was before when it was locking up , if it does , then you know the chain is free from binding at the bottom and still on the bottom sprocket and you were most likely out of time , if this is the case then align the small plug mark on the ignition side with the flywheel , then make sure the counter balancer gear mark is aligned with the case on the clutch side large round plug , you can verify TDC by sticking a long thin screwdriver down in the plug hole and move the engine back and forth using a socket on the balancer bolt and the screwdriver should be at its upper most movement and you should see the mark on the flywheel at that time , then confirm the balancer mark , then install your cam tower while pointing the cam lobes to 2 o clock position facing rear of engine , the 2 marks on the cam sprocket should be aligned flat with the head , then install the tensioner after resetting it in , then spring the engine over by the kick start and see if it rotates several revolutions , and end with the cam lobes facing 2 o clock and recheck marks , if everything is still aligned , you are in time and should not have any lock up now , replace the plug and start the bike ....
Report back with findings