You cant fix it with the motor together. have to take it all apart. You can get that welded, but you have to have it all apart and clean. a non o-ring chain is fine. i have one and there are no problems. take care of the thing.
if you do not have the rear chain guide all your chain is going to do is fall off. when you jump, or ride rough terain, and the swinger drops all the way down the cahin become really loose and weill just jump right off.
i had an o-ring chain break on me and it cracked the case infront of the sprocket. i Just used JB weld to seal it up. have yet to have a problem. i hade a good size peice missing out of the case.
the top and bottom rollers on the blaster are just to prevent alot of chain slap. there is also a white plastic "roller" on the swingarm (where the long bolt attaches it to the fame" that spins,but that is more to proteck the swingarm.
just make sure you have the rear chain guide. its white and plastic. that is number one, and dont buy stock OEM chain rollers. they are crap. spend the money on some from TM Design or a company like that.