The Russian phrase translates closer to something like "By us, there is a motor" instead of "we have a motor". English doesn't really have a separate dative case to show possession, so there isn't a good direct translation. We need the dative pronoun (plural first person) though, instead of nominative, so "нас" instead "мы".
I am not sure what you mean by saying "to roll an r" but I try to help. If you are Brit you probably say R with your tongue located at the bottom of mouth. To say R like a Slav or Arab you need to keep it at the roof of mouth, and air which comes out from you need to make tongue vibrating. Tingue left and right side is sticked to roof of mouth, and top is vibrating like flag on the wind. Unfortunately is extremely hard to lern that if you are Brit. If you achieve that- you will be the man! ;-) if you want to practise I suggest trying to pretend a motor by trying to say very long rrrrrrrrrrrrrrrrrrrrrrrrr.