BILL J. - If I remember my Kohler K-series manual correctly, the K91 engine is it's own little self, 4 HP, nothing interchanges with other engines. K141(6-1/4hp)K161(7hp)& K181(8hp) I consider "Small Blocks", parts swap back and forth easily. The K241, 301, 321, 341, and even the 361 OHV I think of as Big Blocks, lots of parts swap back & forth there too, except the heads on the 341 & 361. Lots of extra parts on the 361's too with the over-head valves. The 241 is kinda like the Big Block Chevy 366 truck engine or 396 car engine. They look the part, make all the right noises, just don't make quite the HP or torque of it's bigger brothers but still enough to do a LOT of work and are still rather easy on gas. I've still got the 5 quart gas tank on the K241 that was on the K181 it replaced and for SON's small yard with the 70, a tank of gas should last him half the summer if it didn't evaporate so fast. When I used the 241 in the 72 I'd suck the last drops out of the second tank with at most 5 to 10 minutes of mowing left, so 10-11 quarts to do 2+ acres plus all the trimming. Sometimes I could finish the yard but the engine would run out of gas idling while I blew off the dust by the shop. With the 129 and my home-made 8 quart tank I'd have gas left on the second tank when I was done, quart, maybe a bit more.
Wyatt & I discussed the K241 Kohler a lot one afternoon when he was out here. We figured the 241 was Kohler's first bigger engine and everything was designed perfectly for that engine. When they needed more HP, they stroked the engine as much as they could, and then increased the bore to get whatever HP rating they needed, along with other changes like bigger intake/exh valves, carbs, etc. once the displacement got big enough.
Because the K241 normally only runs about half loaded all the time mowing unless you REALLY push it, they do run a long long time. I got 1400 hours on my cheapie rebuild I did to my 241 back in 1985. I blew snow for several years with it, running it for everything it had, attended 5-6 plow days, and all manner of other yard work for 20 yrs. It would burn 3-4 oz. of oil per tank of gas if I let it loaf, but if I pulled it harder it would stop burning oil. I just got scared of "windowing" the block.
You could tell the K301 in the 129 had a longer stroke than the 241 when the 301 was freshly rebuilt, it would lug at lower RPM's better, and made more HP when wound up, but the 241 made plenty enough HP for a GD w/38" deck. I'm actually surprised the 241 in your 109 runs a 44" deck as good as it does. I tried putting the 44A deck from the 129 under the 72 a couple times but no combination of parts from the mule drives would get that to work.
Be sitting down when you start pricing parts to rebuild your 241. I'm glad I was!