I bought a new gx31 when I started this. it's still going but it's become quite rattly. It ran flat out most of the time, I changed oil frequently. [whatever good motor oil I had, never 10/30]
I am very pleased with it, it's done hundreds of hours by now.
I spoke to Honda when I had hopes for a small production run, initially I was offered dealer rates for the motors but they changed their minds. They didn't want the risk.
I then looked at the little Robin [Subaru] motors. I bought one in a brushcutter, it's great. So great that it didn't make it to the bike. I think it's a better motor, sorry Honda.
They are ideal petrol motors for what we want, if a little noisy. I did try an extra piece of exhaust with a little baffle, it helped but they work so hard...