Its well worth taking the head off to inspect the valves and valve seats, so long as the gasket comes off in once piece you can use the old gasket for now until you sort your problems out. They are still available it it may be that your local mower shop needs to order one though.
If you got flames back out the carby it is also probably with checking out the key on the flywheel, a sheared flywheel key can significantly affect your ignition timing.
