Hey Ben, I would agree to check your grounds and also check to be sure you wired it back up properly. Did a quick search and found a few links about trubleshooting 25hp Kohlers. Here is one.
http://www.lawnsite.com/archive/index.php/t-304862.html
Apparently 28+ volts should be coming from the stator which I assume you already checked for. If you are getting nothing out of the new regulator, I would assume bad ground or incorrect connections. Have you tried putting the old reulator back on? Maybe you got a bad one????
If you have an electric clutch for your mower deck, turning it on and off while the engine is running might help you measure voltage conditions under load.