Depending on your mechanical ability, replacing the head gasket isn't particularly difficult, but it needs to be done right, if you want the new gasket to last. It is a good way to get your feet wet learning to work on engines though. There are a couple of bolts attaching the upper tins to the head, and 9 bolts on Kohlers up to 14HP, and 10 on the 16HP Kohler which bolt the head to the block. The head bolts themselves are Grade 8 bolts, and if they need to be replaced, be sure to specify that. You should also borrow or buy a torque wrench, and have a gasket scraper at hand.
Replacing the gasket is pretty straightforward, remove the bolts holding the tins, then remove the head bolts themselves and remove the head, taking care not to disturb debris and let it drop into the cylinder bore. While the head is off, it is a good idea to decarbon the head itself, the piston top, and the area around the valves. Bring the piston to TDC before doing any of this work.
Examine the gasket, you should easily see where the gasket blew out, and also check the mating surface of the head for any damage. If the engine was run hard and was allowed to continue to run after the gasket started to blow, the hot gasses can burn a channel through the head, turning into scrap aluminum.
The gasses blew out the bolt hole here.
The gasses blew a channel through the mating surface of the head. You will need a skilled aluminum welder and some machine work to save this head.
If you want to get the new gasket to last, you should make sure the head is flat. Place the head on a flat glass plate, and try to place a .003 feeler gauge between the head and the gas plate. If you don't have a set of feeler gauges, a single sheet of copier paper is about the right thickness. If the head fails the feeler gauge test, flatten it by wet sanding it on a piece of 320 grit Wet or Dry sanding paper placed face up on a flat piece of glass. Once it is flat it is ready to use.
Install the new gasket and snug up the head bolts finger tight. Use the torque wrench for final tightening using an alternating pattern, so you don't tighten all the bolts in one location. Tighten the bolts to 25-30 foot pounds, or 300-350 inch pounds. After tightening the bolts initially to the specified value, wait a few minutes and go through the sequence again before buttoning up the rest of the job.