You can clean the resin off the knots and surrounding wood with white spirit, than when dry, use a external wood glue to re-fasten the knots in place. Any gaps can be plugged with putty, wood filler or the good old fashioned saw dust mixed with glue.
It's not a skilled job, any careful person, should be able to do. Make sure that the edges are true. If necessary I would use the sawdust/wood glue mix, make it a bit high, than when dry, sand down to the right level.