I'll jump in....
I'm one of the guys who isn't "in love" with the idea of a repower for the older machines. Basically, it's not a matter of not original, as it is it just doesn't make sense; financially, or reliably.
The Kohler K series is THE MOST durable small engine I have worked on. I akin it to the old Fairbanks-Morse in that is just so darn reliable. Parts are very cheap, and they are soooo easy to rebuild. Why would you want to get away from that engine?? You can't repower it as cheap as you can rebuild a K. Even if you pack the engine up, ship it to me, and I go all the way through it, it's cheaper. The engine's that are close enough to repower with are not "direct bolt in's". They will require, (for the most part) a lot of fab work. That and, no one makes any ready to buy kits to change to a different motor.
Now, when you get into the spread frames and they used the twin cylinder engines.... that style of motor is pretty similar to so many different manufacturers set up's that it makes repowering easy. For the most part, the crank height is the same, bolt pattern similar, PTO end position similar enough that it makes repowering a breeze. You might have to drill some new holes and shim the base or the PTO to get it in the exact spot, but it's really pretty minor. That and, the KT and Mag Kohlers and the Onans just aren't economical to rebuild at all. Parts are expensive and hard to get. Plus the fact that they just weren't really all that well built to begin with. (Which sounds funny from me because I actually like the HO twins better than the V twins.) For these tractors, repower is the best available choice.
Maybe that makes sense, maybe it doesn't..... but that's my
