Everything I have read says unleaded is fine in these engines. I got 180,000 miles out of an engine, with no issues whatever using unleaded. Being a sidecar the engine wasn't getting an easy time of it either. On the other hand I ran a similar age bmw and that did need lead replacement. I fed it redex lead replacement and found a very slight inprinprovement in mpg and preformance, after reading up on it I did start missing out redex additive every so often and never had to re set tappets which would be a tell tale of valve seat recssion, I belive it is only normally an issue on bikes or cars not built to unleaded speck, and very often only manifests if the vehicle is thrashed, though some engines are probably more vulnerable. A lot of opinions I have noted will say, run unleaded regardless, use the savings to get unleaded valve seats inserted if it becomes an issue. Definitely a sound premise for a classic that gets used a lot. But as has been said, a moot point on the sohc hondas as they are fine on unleaded.
