Re: Chain problems with eng swap..???
you could check and see if the main shaft bearings are seated in the housing of the engine there is a lip that the bearing bottom out on and yours might not be seated or the shaft might have been machined a little off.
I have noticed that the rear mount on these engines have a little off set on the aluminum block that bolts between the engine and bike frame see if you have that on so the off set is correct.
I've found that the rear sprocket will allow more side to side wobble and the chain will stay on but if yo get the sprocket mounted where the wobble is up and down it will make you have a bad day as the chain will go tight then very slack. Just some things to drive you crazy.