"V" twin Vanguard engines are to rebuild! The discoloration on the crankshaft is burned oil. Take it to machine shop and have the crank polished and inspected and the bores checked for size. Metal transfer typically occurs in a lack of lube situation. If you can reuse the crankshaft, make sure the oil passage ways in it are spotless! If you go with the repower, consider Sam Mac's suggestion.