I'd say go for it. I've got a '68 Trail 90, which is broadly similar (all the Cubs and Trail bikes of that era were sorta based off the same motor, with minor differences depending on model). My reasoning it twofold-
1. If the Cub is like the Trail 90, they are getting super-collectable. So the price, for what you get, is quite reasonable. For comparison, a decent Trail 90 will run you about $1500 these days.
2. Honda made a MILLION of these things. While getting your specific motor might be a challenge, the little one cylinder engines show up all the time. There's usually a small pile or two at swap meets, and people like to give them away cheap. You may need to do a bit of swapping and building to get a complete engine correct for your bike, but they are dead-simple to work on.
They're also fun as all get out to ride. I think I have more fun on my 90 than any of my other bikes.