Based on what you say about the backfiring you should go to the resource board and get the Kohler manual. Here us a direct link:
https://www.onlycubcadets.net/forum/...ead.php?t=4646
I would get a carburetor rebuild kit. You can find rebuild instructions here:
https://www.onlycubcadets.net/forum/...ead.php?t=4376. I got a kit on eBay.
My 1450 had a hardware store governor spring on it when I got it. I replaced it with a genuine Kohler spring I got from one of the board sponsors. Check the links at the top of this page or PM me for help.
As far as the thrown rod goes these cast iron blocks can take more abuse than aluminum blocks but I have seen holes in them too. You should disassemble the engine to see what you have.
Sometime in the late 1980s I was on a neighbor's 122 when it just stopped running. The engine spun down slowly and cranked over fast when I tried to start it. I towed it home with their 1000 (which I now own). When I tore the engine down I found the rod shattered in pieces like glass. After 20 years it catastrophically failed. That was my first experience rebuilding a Kohler that started a true appreciation for these engines. The point is until you get into the engine you don't really know what you have.