Strip the block and take it and the crank to the shop.
They will/should tank it, fully inspect it, measure the clearances and let you know what need to be done and what size piston and rod to order.
Most kits I have seen and used usually come with everything you will need including new valves.
IIRC the guides are only about $20 more if needed.
I'd also take the valves and they will/can advise as to the condition and whether or not they and/or the guides should be replaced.
That's what my shop does. your experience may be entirely different.
Good luck
Note: Do not forget the shim behind the governor, take it out for safe keeping as well as shims that may be on the crank and cam.
Here is a link to the Kohler K=series service manual
https://resources.kohler.com/power/k...df/tp_2379.pdf