When I was a kid I had to replace a gear in the hi-lo box of my 1970 ATC90 and I found it very easy to work on. Seemed a very simple setup that even a 13 yr old kid with no mechanic knowledge could fix.
As for the guy who wanted to swap out the engine on a '64 Honda C100, there is a bike on Hoopers site that looks to be close to the same as what he mentioned with the same work done but on a 1961 Honda.
https://st12.flashecom.com/hooperimp...1961_110cc.JPG