I just replaced o-rings on my cub. The most time consuming part was draining, carefully, the fuel. Took 5 minutes to replace the o- rings. Coat the rings and shaft lightly with fuel lube.
Looks like the same valve I have. Here is a parts breakdown for my valve. If you have an industrial seal shop where you are, they can match up the MS number. Its a good reliable and simple valve, but if it needs to be replaced, most aircraft supply houses have them for around 40 bucks. They come set up with L, R, Off, Off. If you need both position you need to drill a hole in the plug. The Dakota Cub build manual gives good detail about to do it.
If it was my plane I would not waste time on that valve, it is not the valve shown in the SL 944 above, if it was it would be a simple fix, two orings and you are done. I have a valve I never used after buying it on ebay, its an Allen, I drilled the spool out for BOTH position, I will sell it for 150. They are 230 or so new, but not a both position, only left right or off.