I measured the new pulley....and calcuted the diameter of the stock pulley so I don' have to take mine off.
The 25 tooth pulley has a circumference of 347mm as measured with a cloth tape, and the calculated diameter is 110.45mm. If I calculate the reduction from 25 tooth to 23 tooth it provides a circumference of 319.24mm, and a diameter of 101.61mm. This makes an increase of the new pulley radius of 8.84mm (0.1738").....or just a tad less then 3/16".
The pitch of the teeth appears to be 13.88 mm - so getting a belt with 2 more teeth would lengthen the distance to the rear wheel by the distance of one tooth (13.88 mm or 0.54")....so the longer belt would move the rear wheel back (0.54" - 0.1738" = 0.3726")....or about 3/8".
(Update: I just read a following post where the tooth pitch is 14mm and not 13.88 as I calculated....and I suspect the difference is the diameter of the belt on top of the hub....or the innacuracy of the cloth tape).
I measured the hubs on both the stock 23 tooth and the 25 tooth, and it is necessary to remove 0.368" from the front of the hub and 0.183" from the back of the hub. I checked my measurements as it was hard to get a good reference point with the calipers I have....and the to distances checked within 0.045".....so that is my accuracy error and when I get them machined I will take both pullies. I have a friend that thinks he can machine it on a lathe....it will take carbide bit inserts and from what I have read.....it takes a few of them.