Saving Old Seagulls

I've bought a 73 Silver Century Longshaft Plus off a local, to power my '64 Silhouette (which has come on a bit since the picture). I live in a coastal location; but its not a local engine. History is mostly unknown; only know it was bought at a boat jumble two years ago and been in his shed since. It starts easily, is all there and looks exactly as an unrestored ex-shed seagull should. Fuel tank remarkably dent free. Proper string supplied.

Did the recommended checks, wiped it down, oiled and WD'd. Set up in a test bin and ran the engine as much as I dared, with a good rev or two because the blocks a long way up; but nothing appeared from the outflow and things started getting hot so I shut down.

Block looks solid, no visible cracks and flushes clean and easy in both directions. (Thanks SOS for the tip; mastic nozzle on the hosepipe). Engine starts and runs. Water 2" above exhaust. Prop drive disengaged - does that make a difference? Don't think it does but I'll mention. Says on here that it won't be the impeller unless someone's been a numpty.

Next step, unless collective wisdom suggests otherwise, means clear the bench; get the tools out. If I read it right the sequence should be:

Foot off, check the impeller's there intact and not upside down
Check pipes and waterways are clear and/or attached as they should be
Poke about in holes with a twisted stiff wire, mine for gunk, hope for an obvious blockage in the waterway
Reassemble, touch wood and try again.

Or is that too optimistic, its bound to be a block full of crud (if so, hows the water getting though? Through the bits that have rusted away ?)

Perhaps better to man up, get the head off and see what's inside. What do you recon?

first thing, remove the head and give the water ways a really good clear out. reassemble and try again. note though that water flow at Rickover is often quite low or nothing, so test in the tank at a higher ticker over, with the prop removed.