I am running a 16/45 on my Hawk 250. Increased my top end to about 63mph but a very comfortable 55-57mph w/o making my engine scream for mercy. I did not lose much at all in 1st, and 2nd+ did get taller in a much needed way. I mostly trail and the paved roads I am on do not exceed 55mph so for me this config works very well. I agree though, it was pretty difficult finding the 45 tooth, finally found on Ebay and it was shipped from UK. My rear sprocket part number is JTR269.45 if anyone is looking. Perfect fit and was already painted black which was nice. Also, this 16/45 configuration did not require any chain adjustments.
https://www.ebay.com/itm/303796685238