Thanks Tony, I'll investigate more next time I'm onboard. Major jobs like taking the pump/thermostat housing off can wait until winter. There's certainly a cold return from the bottom of the heat exchanger to the pump, but not a 'top hose'. I guess since the header tank and heat exchanger is a single unit bolted onto the cyl head and incorporating the exhaust manifold the water flows internally, no hose needed. The only possible take-off points I've found are both on top of the pump housing. It's quite a small engine, often used to drive a generator, pump or refrigeration unit on a truck, so it won't ever have needed a take off for 'cab heating'.
On a related matter, come winter should I close the 'sea cock' and try to drain the cooling system? The engine side can have anti-freeze like a car but the raw water side could freeze?