okay first off i been exactly where you are now... call brandon or rick at repower specialists 800 700 9501 (
http://www.repowerspecialists.com)... dont even bother with all the model numbers, mfg serial #'s, and engine codes.... hardly anyone has info on that old an engine... its just a headache...
here is some info:
1. yes the 25 kohler command pro works fine... but if i could do it again i would have bought the 30 hp....
2. the new kohler will sit a few inches lower so you will have to build a custom mounting plate, buy one, or dont use one and just buy a different belt... (mine took a 72 inch quad v belt) if you buy a new belt you will have to adjust your static lever to engage the disc... youll also have to drill new mounting holes... and the exhaust will need to be slightly modified to sit over the pulley cover....
3. use the old pulley on the output shaft of your old engine... it will take a good puller to remove it cuz its been on there so long... mine moaned and groaned like an old whore before it finally came off... i think the shaft is 1 1/8 by 4 so when you order a new engine get the same shaft size...
4. at repower specialists tell them what chipper you got and they will find my feedback in their system....
5. after the swap out... in my free time i rebuilt the onan and have it in the shop as a back up... you may want to consider this... (mine had piston blow-by and the ignition module was shot...)
6. if you have anymore questions let me know...