Did you check the valve clearances and/or verify that the cams are installed correctly? If the seller is considered trustworthy, then this shouldn't be a issue if they were selling a known good running engine. But generally a good idea to verify and establish a baseline of known settings.
Also, depending on how long the bike was apart for the engine transplant, the fuel may have fouled the jets or stuck the float. If there is no fuel in the float bowl, then the needle is stuck in the seat.