PJ I am not as experienced as some with PTs but I have a lot of experience with the 25 hp Kohlers, which I believe you have. They are on both of my mowers. First of all, the mechanic I use all the time and trust said that those engines are not worth rebuilding. He said he has rarely ever seen one not have problems after being rebuilt weather it was rebuilt by him or someone else. Once you dig into them something is just not right. My experience has been with a burnt valve. I had another engine with bad compresion on the other head so I swapt the heads. With in a few months it developed an oil leak. I took it to him to fix the leak and he said he would do it but it could easily come back. It did with in a 100 hrs of use. That is just my experience, take it for what it is worth.
My vote is if you are on the fence get a new motor. How about the fuel injected 28 hp Kohler. From what I understand it is the same engine you have now with a different cam, compression, etc. So it should fit. From talking to Scarg on the phone(liquid cooled 29hp Kaw motor) he said more hp made a huge difference in his machine.