I like it too. And sorry I'm really bad at appraising values. It's cost me a lot over the years.
The engine seems to have an ever so slight lean (3rd pic) to the left.
That looks like a very solid mount. Possibly the jig shifted when notching or welding the mount bar to the down tube/seat tube? Or when fitting the short tubes to the mount bar?
Even not perfect, I like that mount! Seems adaptable to other (read GX50) engines, too.
Ted