RE: pipe resistance
I would run what you have and keep a close
watch for manifold leaks.
Once the leaking starts - the tuned pipe
performance is interfered with - the engine
cannot run normally. Also the exhaust gases
start flowing by the o-rings and whatever is
left of them gets damaged more = the leak gets
worse.
I suspect that the best way to resolve this
is get with boaters in your area. Compare
your hardware with theirs and see/measure their
o-rings. These are "special" o-rings in both
size and composition.
If that is not doable - just order a new manifold
from ZippKits or any of the fine distributors
you have read about here like Maston's or Gizmo.
If you get a new manifold - before you run it
stop and carefully measure the OEM o-rings. This
will be your best guide to finding replacements that
fit and handle the heat.
These engines require very little in terms of
maintenance but keeping a tight exhaust, making
sure the carb/fuel intake is not leaking, occasionally
checking torque on the cylinder screws, avoid alcohol
blended fuel (at least drain the fuel after running)
and don't try to see how long a spark plug can last
will keep the fun going.