Being you aren't the most technically advanced person, Perhaps you can buy something that is already done.
I have a restored Sears Puddle jumper I'll sell you. It has front and rear shocks, torque converter and a 10 hp Briggs. Enough power you have to be careful taking off as it will wheelie and throw you on the ground. $1000 ready to ride.
If you want to do it yourself I would suggest something with a front and rear suspension and a strong frame. Colemans aren't super strong in the frame department.
You can build up a 212 predator to 20 hp. Of course you'll need methanol and a wheelie bar for that
You can also build up a 301 or 420 Predator provided your frame has enough room for one.
Briggs makes a 14hp World Formula engine that would work well, but pricey.
An old Tecumseh Star race engine would do well too not to mention a motorcycle or snowmobile engine. My suggestion would be to stay air cooled as water cooling adds a lot of complexity to the build.
Danford1