Cleaning out the engine water passages is simliar. Plug the water inlet on the side of the engine and fill up with your cleaning solution, and let it soak for several hours. When it comes to flushing out the crud, it's a good idea to use a device like this that will shoot compressed air and water through the passages. It's made with a rubber plug that fits the water outlet on top of the engine. A piece of tubing with a hose fitting puts in water, and an air nozzle shoots in compressed air. A small hole though the plug allows air to escape and lets water in to fill up the passages. The flushing is done like this.
Another route of attack is done with the water passages dried out. Use a piece of steel cable with a frayed end. Put the other end in a drill chuck and use it as a rooter, shoving it into the freeze plug holes and other openings to knock loose all the crud you can reach, then blow out all the loosened debris with compressed air or suck it out with a shop vac.
A less costly measure is to have the old radiator recored. That's what I chose to do with the brass radiator in my 1915 roadster. Cost was one factor, but my main reason for that choice was that the original tank was still good, and I like the original better than the tanks on the new reproduction radiators. A new core allowed me to keep the original tank, and it saved me a few hundred dollars. The recore cost me about $800, and a new brass radiator is over $1200.
Shops that recore old radiators competently have become scarce in recent years, but there are still some. The one which did my 1915 radiator is:
Toms' Radiator Service
2341 184th Rd Ste 4A3,
Sabetha, KS 66534
Tom is in a very rural area, and farmers depend on him to keep their equipment operating, so their work takes priority and he works on antiques when he has time.
332 South Paul Avenue
Bluford, IL 62814
500 Linne Road – Unit I
Paso Robles, CA 93446