Honestly, I didn't know as much about valve adjustments until the last few years, at least in terms of stuff that requires taking the engine apart a bit. For all I know, they were adjusted at some point without me knowing or remembering. I had it in a shop in Seattle once for something that was hard to figure out and they tossed a number of things at it before it got fixed.
Really, the only reason to adjust the valves at this point would be to quiet it down and maybe gain a little power back, but from what I have read, these engines will last a long time and are happy with noisy valves. I can live with the noise rather than risk getting one of the valves too tight and burning it out.