Hi V,

correct the HiFlo Filter HF112 fits perfect. Its available on Taobao http://item.taobao.com/item.htm?id=8470335355
But quiete expensive there, I bought in Germany and it was less then 2EUR, I remember.

For the airbox - have a look at the Airbox of the X2, it looks quiete decent. I will take some pictures.
It's "copy", mean inspired by some KTM, so the filter can be replaced with decent european stuff, if needed.

Yes this engine has "pep", mine has pep also in lower revs, probably due to the gearing.
What sprockets are on the bike? I hardly get over 100km/h, I guess they are geared totally different.
I dont have a tachometer, so not sure what revs I run.