Check this out, I fount an 18t sprocket for the DF250RTG
I have no clue if the counter shaft is 17mm or 20mm but they have an 18t front sprocket in both 17mm and 20mm.
I'll get back to yall when I measure my counter shaft with my caliper measure, here is the link to the sprocket.
It will work with the vennom x22
https://www.ebay.com/itm/For-428-Cha...kAAOSw5T1c-AoY