Here are some cheap upgrades you can do to your 12hp to build more..
None of these are in order, although some should be done at the same time
Durability
~OHV Rod, rods out of a 14.5 horsepower ohv are known for handling higher rpms.
~Billet Aluminum Counter weight link, know to break under ungoverned condition.
~While you have your engine apart, check out your cam lobs, crank/cam gears, and lifters. If all is good, reinstall with some motor oil on the sliding parts.
~If your crank has a nut on top, swap to bolt.. its stronger. I run a nut and its fine. But bolts are always stronger.
Power, this can get pricey, depending on what your want out of your motor.
I'll get the cheap stuff out of the way.
~Port & Polish, better flow = better performance. Some people like to leave the intake a little rough. I personally like both intake and exhaust smooth as possible.
~Shave Brows on block = Helps with flow and compression.
~Shave the head 0.20 = more compression, some will argue this as most think everything is flow. While Flow is extremely important, compression is torque.
~Dual valve springs, or put another exhaust spring on the intake, which isn't much of a difference. But better than nothing.
~Decarbonize everything. If your aren't lapping valves, dont mess with them. Easy way to cause a leak. I personally like to use a gasket scraper to clean the head then sand with 500 grit.
~E3 Spark plug, some like em, some don't. You decide.
~+4 degree timing key, Dont expect to run lower idle smoothly. But higher rpms, you'll be cooking.
~You want to have a tach, keep rpms below 5-5.5k..Can be bought on ebay for 15 bucks for a cheapy.
Now we will get expensive.
SS Valves, Custom Cam, ARC Flywheel, Dual Valve springs, Billet lifters, Billet spring retainers, get her bored out .30, Custom carb by ec.. that will get you scooting.. there is more, but I cant release all of my secrets