Now that you have the head off, it is much better to just remove the keyhole valve retainer and take the valve out, so you can clean the stem properly. Of course you can only do this if you can push the retainer down far enough on the spring to get the retainer to push off sideways. You probably can: valves always stick in the fully-open position, as determined by the cam lift. There is always some extra movement available before the spring is fully compressed to its solid height.
If you can't get the retainer off, you will have to tap the valve head very lightly indeed, with a piece of wood, accurately in line with the stem axis, until it moves up just enough to enable you to get the retainer off.
In the picture you can see that the stem of the valve looks pretty clean, so you will most likely find that the first light tap on the valve head unsticks it and makes it close fully. Now that the cylinder head is off, of course you should remove and clean both valves, stuck or not, and lap them in.
The chamber looks oily, so it seems you will need to replace the rings. Check the ring gaps and bore size before you order them, of course. Unfortunately GXV140 rings aren't as widely available as GXV120 ones, and are likely to cost considerably more. I've bought good chromed GXV120 rings, with chromed steel rail oil rings, for $11 including postage - but the best I've seen on GXV140 ones is nearly three times that.
When you open the crankcase, please post a picture of the camshaft. You will be aware that the self-propelled drive is by a pair of helical skew gears, one on the camshaft and the other on a special self-propulsion output shaft. That means the self-propelled models have a different crankcase cover to suit, but the same self-propelled cover fits all of the crankcases. I've put a GXV120 one on a GXV140, but haven't yet put one on a GXV160 - but I think it's the same. The reason I'd like to see a picture is because the GXV140 normally has a plastic camshaft, driven by a plastic timing gear. I can't imagine anyone trying to use a plastic timing gear to drive a self-propulsion mechanism, so I'm expecting to see a cast iron camshaft, which the GXV120 and GXV160 have on all models. It would be nice to know if the self-propelled GXV140 actually uses the camshaft from the self-propelled GXV120 - we might be able to tell from the identification markings.
