'79?....antique?......I prefer "well run in".
If it doesn't work there are plenty of places out there with skips - er I mean lots of old engines breaking for spares with suitablty sensible prices. Post back if not, I have a little list.......
Not sure how those 'boxes operate, but there's a high chance it may be the dog and the keys are worn (I had that with the '72 25HP. Was fine until you opened the throttle!) £30 and a new (maybe replacement would be a better word!) geartrain later and it's run fine for 10 more years.