CHARLES - Increasing the piston stroke on a K241 requires a different crankshaft. The K241 is a short stroke engine, only 2-7/8" stroke where the K301 (12 hp) K321 (14 hp) & K341 (16 hp) are all 3-1/4" stroke. The crankshaft from those engines pops right into a K241 but requires a different, shorter connecting rod so the piston doesn't extend out the top of the block. The stock K301/321/341 rod should be used.
There are longer stroke crankshafts available at much higher cost but little things like the camshaft, the bottom of the cylinder walls inside the block all get in the way of the conn rod and crankshaft throw and reguire modification. If I remember correctly, anything over 3-1/2" stroke requires major modifications, meaning time and much more money. In a K241 block, going from a 3-1/4" to 3-1/2" stroke crankshaft only adds a bit over two cubic inches, from 26.961 to 29.035 CID.
For stock class pulling starting with the K241 is not the best engine, most clubs allow the longer stroke K301 12 HP engines to be used which drops right into your CC chassis as long as you use a CC spec block.
Having run K161/181/241/301/321 engines for thousands of hours I can say that there are definite differences in the ways the engines perform when under load. The K301 & 321 engines tend to hold RPM longer when pulled down below full load RPM where the K241 will lose RPM faster because of the shorter stroke which reduces leverage on the crankshaft. There are other things that effect engine performance like rod length verses stroke length, namely "Long Rod" engines tend to perform better under high load conditions than "short rod" engines. It all has to go with Geometry in how the longer rod positions the piston in the cylinder relative to the rotational position of the crankshaft. The "Long Rod" engine parks the piston at the top and bottom of the cylinder longer than the short rod does and allows cylinder PRESSURE to build much higher and allow the piston to exert much more force on the crankshaft. But since the crankshaft centerline and top deck of the block are "FIXED Dimensions", the only way to increase rod length is to use speciaL custom pistons which have the wrist pin bore machined higher in the piston, which effects the area of the piston where the piston rings go, the rings need to be closer together and raised up on the piston which effects ring sealing as the piston rocks in the cylinder when the piston is at the top & bottom of it's stroke.
There's a TON of great SCIENCE involved in tuning engines for high performance, and you and your Grandson will soon learn that engine performance is ALL about COMPROMISE. You can increase one design aspect of the engine design for better performance but will decrease performance in another way. And the Kohler being a Flat Head design engine makes those compromises MUCH more difficult to chose.