Zero compression, oil in the air box ... I'd bet more on the intake valves. Jumped time, piston meets intake valves, jabs whole in the piston. While you've drained the oil, I'd check the timing and make sure they are set correctly ... might aswell check the balance shaft chain tension to. If the timing is off, I'd pull the carb and look down the throat of the carb and intake port of the head. I've seen where one of the screws on the butterfly valve in the carb came out and ruined a perfectly good engine. Basic checks will get you pointed in the right direction though ... fuel, spark, compression, timing. Compression loss can be a few things but it will 8/10 times be internal ... bad head gasket or wrong timing is the other 2/10 I feel. Let us know what you find