A larger carburetor throttle bore will help an engine produce more power. Chances are, if an engine is losing power, the timing isn't advanced enough. Check for a worn points lobe on the cam, too. The secret to gaining more horsepower and torque is high compression. This is done by installing a steel flywheel with no cooling fins.
Gas cleans good, but it is extremely flammable, making it too dangerous to handle.
If you've been wondering what type of motor oil to use in a garden pulling tractor engine , it's safe to use ordinary SAE 30 non-detergent or detergent motor oil. But if it makes you feel any better, it's safe to use SAE 50 oil instead. They could thin out too much and cause excessive wear, resulting in damage to internal parts. I've used SAE 30 at times then SAE 50 in my two Super-Stock tractors and have had good results with both oils. If you don't mind spending a few extra bucks, the best type of oil to use in a pulling application is 20W50 synthetic.