oko 26 will give a fair bit of extra power, but may be a bit of a handful with the factory 1/8 throtle, id suggest at least dropping back to a 1/4 throttle..
only problems iv seen with the carbs is the throttle sticking which if its going to happen it will happen straight away so all you need to do is contact DHZ for a swap over item.. having said this its probably only 1 in 100 carbs with the issue..
now im guessing you dont have much tuning knowledge or time to do so, this is the reason i suggest the oko 26mm from DHZ as it will run straight out of the box, any other carb you choose will require tuning which to some people can be a hair pulling experience..
one other thing to consider is that the oko flat slide carbs offer a pretty aggressive bottom end which may not suit FMX riding..
if you can live with that fine but if not then buy a PE26 or 28 or maybe even a genuine VM 26 or 28
28mm will provide more top end compared to the 26mm