Yes, you need a new chain length but not a new chain. Using a chain breaker take as many links out as necessary to make the chain fit, but first, run your rear wheel all the way forward, or as far forward as it will go and still clear the swingarm. Fit the chain and take out the slack. Sorry, but I don't know what the standard length of the chain was. Yes, that size is a good choice, it'll give you a nice balance of speed and power provided your moped is tuned up good.