First guess would be valves and look at cold clearance on the intake valves. Might as well make the job easier and pull the fender liner, air box and carbs too. Well worth it to make sure the air box is clean.
On valve clearance, a little loose is noisy and slight power loss but too tight is really bad for valve life as they run hot. On outdoor power equipment, valve adjustmet too loose can make hard starting as it negates compression release but the EX250 doesn't have low speed compression release.