My DS love making things and has loads of lego, I would like to buy him some extra things so he can make remote control models or models with power functions but not necessarily a proper kit just some extras that he could use to adapt his lego /knex and use as he wants rather than to build a specific model -IYKWIM . I have looked at the lego power functions kit and I think Knex do some power functions too - does anyone have any ideas?