What would make a useful baseline for a modern gunboat?
Short answerA River-class OPV adapted with a 5-inch or twin 3-inch gun would be more flexible than a purpose-built small gunboat.
NotesA modern gunboat proposal begins with the need for a hull useful on other operations. The historical motor-gunboat example in the question is a possible starting point, although the preferred alternative is a River-class offshore patrol vessel. Fitting a 5-inch gun or a twin 3-inch mounting is suggested as a way to add firepower. These are options to investigate, with no detailed fit established. The design preference is for a flexible patrol ship that can undertake missions beyond its gunfire role.