so you actually swapped the engine without removing the compressor and de-gassing the system??
hmm.. does the motor bog down as soon as you turn on the air cond? does the clutch ever kicks in after you press the a/c button? If yes, but turns off again after 3 seconds, you may need a regas, but if it bogs down the engine significatly, or it's burning the rubber belt before shutting down, the compressor may have seized due to lack of operation/lubracation, if the clutch doesn't kick in at all, check the electrical, clutch system etc. do some test and tell us the results for more help.. cheers..