Here are some thoughts for you about the engine color - The later Kohler long blocks (complete engine with tins installed) were painted black. If it has green tins then I would expect them to be original to the tractor unless someone painted the black tins green. The green engine is the correct color for your tractor but black is correct for later tractors (both green and custom color/patios). Since it is a '68 a K301 (same engine used in the 120 FWIW) is also correct for it.
I believe a larger Kohler will bolt in if that matters. There is a member on WFM that put a 16HP K341 in his 140 with minimal modification.
More '68 info from WFM
For the 1968 model year, Deere took on all comers with the all new model 140. Marketed to commercial users, truck farmers, nurserymen, and estate owners, it had the power and features to take on many tasks formerly relegated to larger equipment. Advertising of the day stated that the 140 could be purchased for roughly the same amount as "5-weeks wages of one good handyman". Based on an all-new design, the 1968 140 incorporated several new design features as standard never before used on a Deere Garden tractor:
- Twelve horsepower, air cooled, Kohler model K301AS cast iron engine with Bendix style starter
- Sundstrand hydrostatic transmission, unlimited forward speed to 6 > mph
- Hydraulic attachment lift, with three spools optional (H3)
- Quick -Tach style mounting of attachments
- Electromagnetic PTO clutch for front and center mounted attachments.
- 1 > gallon fuel tank with gauge
- High back deeply cushioned seat adjustable for height and reach
- Live rear power take off, used to operate the #33 tiller.
Introduced as a 12-horse model featuring the Kohler K310AS powerplant with a Bendix style starter, the 140 benefited from a hydrostatic transmission, allowing a seamless transition from forward to reverse travel, as well as static braking. A single large lever on the right side of the pedestal controlled this. The transmission was directly coupled to the engine via a steel driveshaft. A cone style clutch was provided to disconnect the engine from the transmission to aid in cold weather starting. For the 68 model year, this clutch was actuated by a single pedal on the left side of the tractor, which also applied the brakes. No provision was made to force the hydro lever back into the neutral position.
The hydrostatic transmission also afforded a new feature formerly only found on large farm tractors; hydraulic lift. The charge pump on the transmission fed a single spool valve on all models. The H3 models used the power beyond output of the single valve to supply pressurized oil to a separate two spool valve. The three levers on the left side of the pedestal were closely spaced to allow them to be "palmed". Pioneer style couplers were utilized on the front of the tractor to control attachments and an optional rear set of outlets powered a Category "0" three point hitch or other rear attachments. Deere advertised the ability to use multiple integral attachments at the same time, something the other manufacturers could not accommodate. The option of a front blade in conjunction with a rear mounted tiller was a popular choice.