Seems like the rebuilder has done something wrong. Most obvious, does the choke work? It could be compression, it is worth checking, anything over 90psi is OK, if it is low, re-set the valve clearances and check again. If compression is good, clean the carb again, just hope the re-builder didn't fit parts wrongly, or the wrong parts.
Just a thought, fit a new plug first, surprising how often this is a magic cure.