Update with the final fix. New Spark Plug installed but it still backfired. New Fuel Injector and new MAP installed - Problem solved. Had to do both at the same time b/c the unit is so hard to get in to. If I were a guessing man, since I was able to start it up and rev it after installing the fuel injector, that was more than likely the problem but I installed a new MAP anyway. The old fuel injector was pretty ugly / dirty. Hope this helps and thanks to all who chimed in.
y2kbmet's post in 2018 Hisun Strike 550 Backfires on deceleration was marked as the answer