Found this website
Futurlec, The Electronic Components and Semiconductor Superstore and they have the following Part listed at US$32.90
Part Code: GASFLOWMETER
Flow Rate 2.0 - 30.0 liter/hr
Fitting for 3/8" Barb
Suitable for both
Diesel and Gasoline
Displays Both Flow Rate and Total Number of Liters/Gallons
Unit Can be Switched to Display Readings in Gallons
Ideal for Tractors, Generators, Go-Karts
Low
Battery Power Alert
Fluid Temperature: 0 to 60°C
Fluid Pressure: up to 85 psi
Supply Voltage: Uses 2 x AA
Batteries
As A64Pilot stated, you will need 2 units, one for Supply and Return lines each.
Will the flow rate of your Engine being sufficient for these meters?
They also don't state the accuracy, but may be a place to start?
Cheers