If your chain is coming off it is due to them being "toast". The stock roller chain will wear out quickly, as you have seen.
My .02 is to replace them with a high tensile strength o-ring chain and new sprockets in the stock sizes and you'll probably never have to replace them again. 13hp does not warrant a high-tensile chain from the standpoint of strength, either. But high tensile strength is an indication of better materials, so get the highest tensile strength you can afford and it will probably last for longer than most folks will own their bike.