What's actually wrong with the cranks? This mess is going to cost you $50 minimum just to get a turd of a frame running with the same chainline it already has. Also, you will need new pedals, the ones from the one piece rig won't fit a 3 piece crank, so there goes more money. Bracketing the fact that you could almost surely get a new beater for that price, I'm not sure what could be so wrong with a set of one piecers that you would need to do all of this.