Hi There..I posted in the wrong spot so I updated and moved my post to the right location. Sorry on this.
most likely seal around liner but something this old could have electrolysis on a liner put sleeves and replace all liner seals because the other ones are not far behind. Pull the oil pan and pressurize the cooling system you should be able to see the leak. does it have a gear driven water pump if so check it also
Thanks on the reply Dave. No gear driven water pump unfortunately so likely electrolysis which goes back to what I thought originally. Motor seems all original ~ 5000 hrs roughly but i forget exactly.