I would for sure get the k321 rebuilt if your keeping the machine and going to use it hard.
Five or six hundred dollars for a rebuild useing Genuine Kohler parts is far cheaper than a new tractor which are junk plus that old 1450 will still outlast the newer MTD machines if proper maintnance is done.
if you can get the engines to Lebanon PA I will give you the number of the engine machinest who does my work.Pm me if interested.
heres the cost of a repower
http://www.smallenginewarehouse.com/...tegory=1709551
Thats the only direct bolt in I know of and you can't use the side shields because the engine is too wide.
rebuilding the k321 seems the way to go to me.