Mike,
Who rebuilt the motor?
Does your carb say Walbro on it?
One way to find what size carb you is to remove the air filter cover and look down at the air intake. There should be a number looking back at you in the 12 oclock position. Let me know what number is.
Page 6.4 in a kohler Service manual has the preliminary low idle and high idle fuel needle settings for your carb. There a procedure to fine tune the carb in section 6.
In the FAQ section (top of the page)there is a chart for different brands of sparkplugs you can use in your motor. I've had problems with champion spark plugs in the past and don't use them. I use Autolite spark plugs. The one for your Cub is a 216. Sparkplug gap is .035.
It would be a big help if I knew the age/condiation of the battery.
Here is a link to a free Kohler manual if you don't have one:
http://www.kohlerengines.com/common/resources/tp_2379.pdf
The link was found in FAQ section at the top of the page.
14 horse piston in a 12 horse block:
That is ok if you are building a puller. A hydro trans isn't a good choice for pulling. They loose too much power. Puller motor(s) only run for a short time. I will run a every day use Cub 2 to 3 hours at a time.
Reason(s) I don't like next size bigger piston:
1. extra piston weight can throw the motor out of balance
2. cylinder wall could be weaker because of the extra material removed.
3. you will be getting into the softer part of the cylinder wall when boring the motor to the next horsepower size. Softer cylinder wall will wear faster and have oil control problems.