"Unless you attach/affix/weld/bond tubing/piping filled with water directly to the engine, to both heat the water and help cool the engine, as I am suggesting."
Thus voiding the warranty, if not causing problems with thermal stresses that the engine was never designed for. Air-cooled industrial IC engines typically have finned aluminum heads, with very little metal thickness to which to attach water channels. Recovery of the cooling heat is pretty much going to have to be done by the HX method that the OP is looking at. If a water-cooled engine is available to do the same job, then yes, he should look at those (they typically last longer too, though are more expensive and heavier per unit hp).
As far as the OP's question, you will need to run tests for the specific engine and hx combination you are looking at. You need to avoid blockage of the airflow (limiting the cooling of the engine), so tests with and without the hx in place should be done, measuring cylinder head and oil sump temperatures, and any other points the engine mfgr. might ask for. The local engine rep should be able to hook you up with the manufacturer's engineering department, and they may be able to give you advice, up to and including reccommended test procedures for verifying the cooling performance of the engine installation. If you are just doing a one-off, as opposed to being an OEM, they may or may not pick up the phone for you. Good luck.
