Saving Old Seagulls

Hi, when my seagull starts it will run ok until i put it in gear and then it just stops dead, any ideas please

A suggestion would be to take out the spark-plug, put it in gear and turn it over by hand from prop or flywheel.
If its hard to turn over then its a gearbox end problem and the motor is going to struggle, if not, do a compression test it see if the motor has still the power it needs to turn a prop.
Its going to be one end or the other.

Thanks will try that