I would say buy a running bike so you can ride it to see if it runs ok, as in the past I have bought bike engines which you get limited guarantee on, 3 months I think it was. well it took me longer to get round to fitting it. when it was first run there was a god awful noise and turned out to have had major crankcase repairs done but cleverly disguised on the outside anyway...inside there was lumps of weld fouling the gearbox, result was one knackered ZX9r motor and £1200 down the toilet.......live and learn buy a whole bike or one from a known supplier like Andy at AB performance or Yorkshire engines.