Shop vac to get genset impeller parts out of heat exchanger. Pull the "output" hose off the water pump, duct tape your shop vac to the hose, make sure the exhaust "flap" is open (if you have one), turn on the vac and count the pieces you get to match how many are missing from the impeller. Works well on my Onan.
Does it not suck hard enough to be concerned with the water laying in the muffler coming back into the Manifold?