I've got a 2003 YZ450F that I picked up this winter for what seemed like a very good price - $2300. I was riding it last weekend when the motor completely grenaded. Unsure exactly what happened, but a mechanic friend of mine tore it down and it looks like pretty much everything went. Valves / cylinder / crank / piston / crankcase (got mashed by the crank) / probably the head are all pretty much destroyed. What are my options here? My mechanic friend can rebuild it, but I would have to pay him for his time. Any ideas how much this will cost me? Is it worth doing? Should I just part the thing out and look for another one? It doesn't seem like picking up a used motor is easy/common.