
    It is certainly possible, and quite easy in most cases, especially
on two-strokes.  It's very common to do this on bikes where the
cylinder-mounting through-studs don't prevent the cylinder from being
removed with the engine in the frame.
    I did this when I assembled the top end on my Indian, which was
easier yet because it does not have through-studs.  Base flange studs,
and separate head bolts, dontchaknow.
    You have to use a little foresight, rags, duct tape, etc. to keep
clips from falling in, clothespins, hoseclamps, etc. to support the
cylinder while you're inserting the pins.
    (Are these called Gudgeon Pins in the UK? If so, what's a Gudgeon?)

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