How do piston engines deal with sea spray (saltwater ingestion)?
Seaplanes operate from coastal saltwater all the time, ingesting lots of seawater, and they don't do anything special to the engines to deal with seawater ingestion.  The effort is mostly on keeping salt rinsed from the airframe and engine externally and they normally get a fresh water rinse at the end of the flying day.  Some operators may spray nicely atomized fresh water into the engine intake while running it to rinse salt from intake passages and carburetors etc, when they are doing the external bits.  

The other measure is keeping the protective coatings intact, including the paint on the engine.  You really don't want salt on an engine where paint is flaking off aluminum fins or areas of the head, because corrosion pitting results that leads to cylinder head cracks. 

So the main protection measure is, keep the salt rinsed off as much as possible with fresh water, and maintain the protective coatings. Even with all that, coastal aircraft operating daily from seawater year-round often require airframe rebuilds to deal with corrosion maybe every 10 years or so.

In any case there aren't any physical modifications you can do to an engine.