The exhaust is the only thing that’s different
from the 1937 so you have plenty of parts there.
The one rope sheave is a 1936 as you have
pointed out. The spacer between it and the
flywheel is also from a 36 and was only used
that year. Only 1893 twins were made 1936
and few survive. Quite a few parts are unique
to the 1936 like that spacer. I have one that
is mostly correct. Not at home so I can’t provide
pictures. These pictures are from the Waterwitch
website. This is probably the nicest surviving
1936 but is has a couple parts missing. The 1936
twin is the only one that’s uncommon. Later
models are not hard to come by especially
in the condition yours are in.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.