Mr B suggested in comments that a roller vane pump would have lower friction losses than an internal gear pump. Mr B blogs at In the Middle of the Right.
The attached video shows how a roller pump works. Frankly, it would also be easier to fabricate since the the internal dimensions are not as tightly spec-ed as the internal gear pump.
The pump in the video is extra-fancy because it has two intakes and two discharge zones.
Simply placing the rotating element that carries the rollers eccentrically in a round section would result in one-and-one.