Doing a compression test won't do much for you on the Honda 450R's since the exhaust valves hang opened just a tiny bit due to the centrifugal decompression wing that is on the Cam Shaft.... you only get full compression when the engine is running. I'm betting it's the Piston rings for sure, especially if it smokes when you give it throttle.
Now If it was the Valve Guide Seals it would smoke only when the engine is cold (first initial start up) for a minute or so then goes away when it warms up, especially in colder Clements.
I would put my money on piston rings for sure. If you decide to rebuild it I'd get a Piston, Piston Rings (Or a just decent quality aftermarket piston kit) top end Gasket Kit, Valve Guide Seals (usually valve Seals come with good top end kits), get the Cylinder Ball honed (or buy a ball hone along with all the parts if you know how to do it), check the valves while the head is off. I usually recommend replacing the piston when ever doing a top end rebuild since it is a fast moving part.
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