50hp overheating help

[Jeremy1986]

i have the 1973 johnson 50hp and i had it recently serviced for a waterpump and carb rebuild. when i got back the motor was running alittle hot so i installed a new thermostat, head gasket, and a pressure relief valve. when i did the head gasket and the thermostat gasket and new thermostat both area's were cleaned and not blocked they just had a small amount of light corrison. so i cleaned all the water passages and put back together she ran cool in a bucket. i had run for about an half hour and no overheat. so i took her lake last night and park at the dock for 15min still didnt overheat so i took her out for a ride and i was out for about an hour all sudden the buzzer went off and the motor shut off. my spark plug wire was laying against the spark plug it melted to the spark plug. i got back to house and checked compression and she had 90 on both cylinders. she had 180 before i took her out. is this motor trashed or can it be fixed. thanks again guys.