A backfire is fuel ignited outside the cylinders, or an explosion getting outside the cylinders.
How many RPMs are we talking about? How about a compression test?
I had a 1956 Ford wagon that used to explode once and a while - it had a hole in a piston and a spark plug that had a broken electrode - I could push the center electrode up into the plug. It would fire into the crankcase!
Several gaskets were damaged and oil leaked out everywhere.
A cracked valve? I had one on a 61 Mini - it ran fine.
I would try new plugs, easy and cheap to try
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