Why was the bike torn down and not put back together? If you have never worked on a banshee and your basically going to pick up the puzzle pieces and start rebuilding you had better have a manual. $400 isn't to bad if everything else other then the engine is still together and in good shape. Not knowing what else needs to be done to it to get it trail worthy can start going into deep pockets fast. I would say if you have $400 sitting around and time to work on it have at it. I have done this before with a dirt bike and had nothing but headaches and money being spend left and right to get it in the condition I wanted it. Would I do it again? No. I would just save my money and get something that you can hop on and hit the trails. My $ .02
Good Luck.