A backfire will produce a horrible clang/crunch noise in the sprag clutch as the crank tries to turn it the wrong way. Not nice, but I get this sometimes if my Electra-X is playing hard to get when starting.
Kick the bike over slowly with the plug out just to make sure nothing is actually wrong in the top end, then clean the plug and kick over with the plug in and ignition off and fuel off to make sure you have compression as normal. I'd also have a good look at the pilot jet to make sure there's no blockage. After that, fuel on, cold start plunger out, kick the engine over a couple of times with the valve lifter/decompressor open, then ignition on and try starting as normal.
It's unlikely that a failed clutch cable would damage the engine, I've had several fail over the last 8.5 years, the last one on Thursday!
Regards,
A.