There were some things going on with Cox prior to their purchase by Estes Industries. Seems these engine companies prior to failing did some things that weren't the best for the market. For K&B it was the release of their Sportster series. For Testors, it was their Series 21 engines. Cox 074 Queen Bee also was a poor fit. Overall were they bad engines? No. For K&B, the new technology wasn't tolerant of abuse. Engine was easily ruined by simply running lean, and they were heavy. Testors Series 21 were too heavy and cross scavenge technology as good as it is was losing out to Schneurle. Cox Queen Bee is heavy. Modelers were looking for a Tee Dee like offering. Weaker power through reed valve technology plus the heavier weight disappointed the modelling community.
Seems almost like these companies were grasping for straws, and finally faded away.