Hi,
first remove the carburetter, then you need to remove the black bakelite carburetter mount by removing the
3 screws. Now you should see a bakelite frame on which the valves are screwed.
When replacing the valves check this frame for any damage or bending , if so you should
replace it too. The frame should be placed airtight with the aluminium housing of the engine and
the retainig screws locked with lock tite. Do'nt tighten too firm for not to brake the bakelite.
Same story for the carburetter piece, 2 of it's 3 screws serve as cranckcase tighteners so they should be firm, but not to break the bakelite.
Cannot post foto's ( don't like to unnescesary take apart an engine )
Good luck