Welcome to GT Talk.
First questions. What are you working with? What do you plan to do with it?
Like stated above, weight will increase your ability to pull. If you are doing competition pulling I'm sure that you are limited on weight according to your class. There should also be a limit on the height of your hitch. You want to run your hitch at the maximum height allowed. This gives up pull on the tow weight making it easier to pull. Following the hitch height, you want enough weight on the front of the tractor to keep the front tires close to the ground. The higher the front is in the air, the lower your hitch is to the ground. Not to mention the whole control thing.
One way to increase engine power is to increase RPMs on the engine, most stock small engines are governed to 3600RPMs. Increasing your RPMs to say 4200 would give more power with out damaging the engine. Any thing past 4200RPMs will take more $ in order for the engine stay together. In competition max RPMs is usually another rule according to your class.
Or there is the "throw money at it" solution. Take your existing engine and build the snot out of it. Here in the states we have kits that will help you get 13+ hp out of a cheap 6.5hp engine.