If you haven't yet, download the 50-175 Service Manual from the Downloads section. The 175 technical manual is more specific, but the scanned photos are not good in it.
The rotary valve should just slide off the toothed collar (hub). If yours is stuck, try some PB Blaster or other penetrant, or if rusty, some rust remover solution. You may have to try to work something behind it, working from the intake port area. If you have the cylinders off, you can push from the inside of the crankcase through the intake port. Even a suction cup or two might work if you smear a little oil on the surface as there isn't really a good way to grab them.
When replacing them, there is a R and an L molded into the surface at the hub of the rotary valve which has to be lined up with the locating pin in the toothed collar for the appropriate side.