Should just be able to take a Allen wrench set and unscrew plug. Apply PTFE to last of sensor threads and screw in 3/4 of length, If fully inserted it will bottom out on steel mesh of filter giving alarm
If its a salty dog, just do a heavy decarbon on it and run it till it blows. 1977 should have the high ring pistons unless motor has been into. Too many V-4's laying around for pennies than waste coins on this dog. Also its only a 115hp at prop...