It sounds like the simple solution would be to build or buy a .25-size trainer like the Phoenix Sonic low wing ARF, the Sig Kadet LT-25 kit, the Goldberg Freedom 20 kit, or the Great Planes PT-20 Kit and then outfit it with the same E-Flight Power 25 brushless motor and ESC that your Cub came with.

You'll be able to train with the same power setup and fly it with the same battery that you already own.