Did you get your starting/running issues resolved ?? , you never replied back with more info .............
As far as the chain goes , what "modifications" are you referring to ?? , as long as you get a front sprocket for a 450X , and made for a O-Ring chain , it should have a offset option to turn it so it sticks out farther for wider O-ring chains if the rubbing the case is what you are worried about ....... personally I use the DID ER Gold chains (non O-ring) as I feel the O-ring chains cause way too much drag .... , but the key to getting a O-ring chain not to rub is the front sprocket having a offset ....
There are narrow and wide chains , but unless the chain is exceptionally wide , with the correct sprocket it should not rub ...
But to help with your question , here are 2 narrow chains ...
https://www.crfsonly.com/catalog/produc ... ts_id/6991
https://www.crfsonly.com/catalog/produc ... ts_id/3092