I think you'll need to account for viscosity and reynolds number effects. Why not use air? Rotameters are fairly cheap on thebay. Vacuum cleaner, throttle, rotameter, dummy cylinder with valve arrangement.
I just ran into a website with some info about the OPS 1.20 four stroke. I don't know if this is current or just informational. Bottom of the page.
http://www.mantuamodel.com/index.php...elli&Itemid=92
Greg