HVAC tech here. For the following reasons, I suggest the purchase of a replacement system of the correct size: A fixed expansion device (capillary or orifice) calibrated for 12,000 may not produce enough discharge pressure to allow proper operation. Even an automatic expansion device such a TXV still may not have the range to "hold back" enough discharge pressure. If I were to install such a compressor, the existing expansion device would be replaced with a TXV matching the capacity of the new compressor. However, Sporlan, a major supplier of TXV's does not make one in such a small size for R-410A refrigerant. Also, for use in a reverse cycle system, the replacement compressor would need to be rated for heat pump operation, which requires a lower suction pressure. There are heat pump rated 6,000 btu compressors which use R410a. However, a compressor taken from a window AC unit will not likely have this rating. Another thing to consider is the mounting pattern may be different and the piping connections will be different.
Daniel Varholy
HVAC journeyman
'89 Tradewinds 43' MY