I wouldn't say you bought a lemon, just the way it is, these old mowers have done a mountain of work over the years and it is a risk you take in buying one, might be ok,might not. As for finding a good second hand motor, probably as risky as what you have already bought and there is a fair bit of work in changing a motor , always something doesn't fit or is a bit different. New Chonda motor will cost $450+ and that still requires a fair bit of work in fitting them