I wouldn't consider 25C hot. More like 30 or above.
Air-cooled engines run at various temperatures based on several factors, one of which is the ambient temperature. There is a temperature range where the sohc4 engines will run best, and that is the temperature that things like jetting, valve tappet gaps, and all other clearances between moving parts in the engine are designed for. When multiple parts have worn past their factory specs you're going to experience changes in how the bike runs from the way it ran when brand new.
In addition, carburetor and jet problems, as well as valve issues can cause the engine to run hotter than it normally would under a given set of conditions.