Hi Michael,
Great job on the engine and pump. Engine's a good runner!
Looks like there is room enough for a pair of gears and an idler gear between them, if you wanted a more positive drive to the pump. Gears would be more like the real thing too for an old-style engine, compared to an o-ring drive.
If you don't want to do gears, a poly V flat belt and pulleys would give more positive drive and from a little distance would look like a real flat belt found on old engine water pump drives which again would be more to scale appearance.
Flat rubber belts were used in some VCR's – you might be able to find a short enough one – but in my experience they usually don't transmit power smoothly in small sizes. The poly V belts and pulleys are far better. JD