Do you know any details about the conversion? If the conversion was done by simply putting on r134 fittings, evacuating and charging, then I believe this is the cause of the problem. The oil in the R12 system is not compatible with the r134. The system should have received ester oil at the time of the conversion.
You now probably have a serious mess to deal with. The system MUST be flushed thoroughly everywhere, and see that the flush is THOROUGHLY removed from all components before reassembling and evacuating. If you search the internet for a suction side filter, this would be a very good step. By adding a suction side filter, any contamination left in the system from the burn out will not be allowed to wreck the new compressor.