Seems like you purchased a bad motor. I would, as already suggested, take it on the chin and move on. Good on you for being brave enough to open this thread as it can be embarrassing to admit you purchased a lemon but it does remind others to be careful when buying privately.
Not sure if already covered but must have a scrap value? Compared to a lot of motors on the road including the new and old it has not worked out an overly expensive period of motoring.
Thanks for your advise KSLB , I've just managed to sell it for export so have recouped some of my losses