Sounds like i would throw it away and look about for another mower with a good motor on it.
Tip shop, hard rubbish, eBay too but they are usually over priced.
Beauty is they all mount up the same, things mount to them the same.
If you want to know the condition of a used one, Grab a few screwdrivers and a torch, remove the cowling, pop the muffler off and have a look.
