All stock clones i have seen have been 25mm and 24mm valves, no matter if it was 14cc, 18cc or 22cc. Now honda heads were a little different with some having bigger valves like certain GX160's
Those are the ones i was thinking of, they come with the good flattop piston and bigger valves while the UT1 came with smaller valves and a different flattop or dish piston i believe but am not certain. Kart43 could shed some better light on this issue if he would see this thread, he knows alot about the honda engines