Yep, 428. RK sells a chain they call a 428 and it is actually a 425.
I was out when I posted my previous reply, but now on my laptop I have bookmarks for some cools stuff.
http://www.sprocketcalculator.com/ to help you figure your gears.
http://www.jtsprockets.com/catalogue/model/5198 (1979 xr185)
Then you go to amazon and find a seller, my rear sprocket was right around $20 with free shipping (prime) I had to take a file to the center hold and dress it out a little bit from the stamping edges. Basically a perfect fit. The center clip is tough to get off. Get a good snap ring tool. The kind for installing piston rings is good.
The 17t front just barely clears the cases in front. Gives a slight gear change, about 6%.
The 45T rear was the ticket. I can now keep up with traffic, and still have plenty of power for the trail.