Backfire turned bad day

They always run best right before they blow up.

Since you're running nitrous, I suspect you ran lean and cooked the pistons. That will usually mess up your valves too.
Take the heads off. I doubt by some miracle that it is something that doesn't require taking the heads off.

If you want to tip toe into this, do as already suggested, compression test. Pull all the spark plugs, have a look at your freshly roasted aluminum.

Power adders have a bad habit of burning aluminum when they don't get what they want, fuel. I hope it is something simple but that's wishful thinking. It's normal to have oil in your intake runners and air cleaner when the magic smoke goes out the exhaust.