If the word Diamond is cast into the head or block, chances are it is an International truck engine. There was a period of time when they referred to their engines as "Red Diamond" and "Green Diamond" and possibly others.
I don't know what engines were used in Seagrave Fire Apparatus, but if they did not have their own power plant, they could have been Buda, Lycoming, Continental, Hercules or ???? I had a Diamond T truck of that era that had a Hinkley engine.
I would think the first thing for them to do would be to find out what they are looking for. There are plenty of Fire truck guys (and forums, probably) out there who would be glad to help them. Good luck.
Mike