Everything depends on how the bike has been and will be used. A top level pro on a YZ450 would probably need a top end in 25 hours or less, even if only to keep up the highest level of performance. Recreational riders often get more than 400 hours out of YZ450's without ever doing more than oil, filters and spark plugs. And not many spark plugs, either.
Where you fall on that curve is hard to say, but if you're competitive at a B level, it's probably time to look at the top end. Plan on a new piston, rings, pin, new clips, and a head and base gasket. As to the valves, without at least a tear down and visual inspection, it's really hard to say how long they'll go. If they have never needed more than a one shim size correction, they're OK for the moment. Valves that have been adjusted once should be visually inspected as long as the top is off, even if the clearance has remained stable since it was adjusted. Any intake valve that has a bright ring worn onto the otherwise dark gray face surface needs to be closely looked at. If the edge of the wear pattern is "fuzzy" or "feathered" on either edge, you can consider reusing it. If both edges of the wear stripe are well defined, it will wear through the hard coat in a fairly short time after you put it back in service.
Guesswork, but without the above mentioned expensive lab work, which is just a more expensive guess a lot of times, that's the best you can do.