You can look at it this way
1) your Cub was built to be repaired and run forever, there are bearings and bushes every where they are needed and at worst some might need a custom made replacements
2) to get a mower of equivalent quality now days you will need to spend better than $ 15,000
3) to get an engine of equivalent quality you will have to spend $ 5,000
4) if you have been running it for 47 years then how much longer do you expect to be physically able to run it , I turn 69 this year and time is definately taking it's toll on me.
SO it is going to be a value judgement.
I would go for a new engine if I thought that I would be able to use the mower for another 10 years.
And because I never throw anything out, it would then be a case of striping the engine and evaluating the cost of a rebuild.
The older engines are a lot tougher than their modern throw away equivalents so are worth fixing.