I have several OS and Magnum four strokes and they are very easy to operate.
You will need a tachometer to set the high speed needle, this is really not optional.
For my money a 14x6 prop is the right size for a 91 four stroke, I use Master Airscrew K- Series, Scimitar series, and APC ... they all work well, just pick the one that you think looks best on your plane and go with it.
Some people check the valve adjustment before they ever start the engine, I don't, I crank em up and if they work correctly I leave everything alone until there is a reason to check something, I have never even looked at the valves on most of my engines.
I did have to adjust the valves on one of the 91's and it was very easy to do it.
This is a just a FYI thing ... The OS valve adjust kit has the correct feelers but the wrench is not the right size for the nuts on the Magnum.