If it's a stock 50 then your stock carb is a 13mm and the largest carb you can put on it is a 17mm off a crf70. Any bigger and you will never be able to tune it properly. On the stock 13mm carb I just measured the size of the inlet for the air filter and went to my local moto shop and found one that would fit
Also when you put a uni filter on you may need to tune the carby (I did but some people say the didn't need to). My bike is a 2006 model and the needle in the carb is not adjustable so I had to buy some little shims to lift the needle to tune it properly. I got them from bunnings, they're just little washers, I think I added 3 or 4? Anyway this is only relevant if your carb doesn't allow you to adjust the needle settings