1st try shifting your battery and receiver as far forward as possible - also consider a larger battery. You may also want to investigate a larger (heavier) engine and heavier wheels. If you have to add weight [] , make it "useable weight" whenever possible.

In addition to Higley's Heavy Hub, DuBro sells "crankshaft" weights. They go under a spinner and are used in place of the washer under the prop nut. SIG also sells weights with holes drilled in the center of them - great for attaching to the motor mount. Just use a longer engine mounting bolt.