Well Duane, it appears the motor is new-old-stock, correct? This would explain why the price is so high.
The other thing I'm noticing is, this motor appears to be one that runs off of an internal coil that is mounted under the flywheel, and not an external coil. I'm assuming it would require a standard starter, and not a starter/generator? Your round fender should be set up for a starter/generator type, and not the standard starter type. I suppose your tractor could be converted over to a standard starter, however you would have to change your wiring considerably. Not sure how an internal coil set up would work with your round fender voltage regulator.
That's a lot of money to shell out for something that may not work for your desires. I'd look around on eBay to see if I could find a decent used motor first, or just keep looking on Craigslist to see if the right motor will ever turn up. Even if you have to rebuild a used motor, I would think it would still be cheaper in the long run. Plus, the value of your tractor would be worth more if it had the correct motor or starting set up. Just my opinion, but it's your tractor and you can do with it what you want.
Thanks for the replies. I also noticed the ignition was not the same. I tell ya these k181s are hard to come by in my area. I'll think I will keep looking around. My resto is kinda stalled out till I find a motor.
On a different note, I need to know the size of bolt that attaches the mower pto clutch bracket to the frame. The bracket that the actual foot pedal mounts to.