Small Engines (Lawn Mowers, etc.): 12hp B&S wont start, spark plug wire, plug hole

Question
I have a B&S 12HP model 281702 type 0020-01 code 8802 1811 on a Snapper 281011BE(engine is a replacement) engine started, when I engaged the blade or put it in gear it stalled. It did it 3 times, on the third try the spark plug wire smoked and burned. after that of course it would not start. I replaced the armature-magneto(adjusted to .010) and still it won't start. It has a spark, and when cranking a few seconds it backfires through the muffler. I also put a teaspoon of gas in the spark plug hole a few times and tried it,  same results

Answer
Was the electrical connections on the old engine the same as the new engine?

Have you checked the flywheel key to see if it is sheared?  Larger engines rarely shear flywheel keys but I always check.

Did you just install the replacements engine?

Let me know if the key is sheared?
Eric