For calculations you will need the following data first: prop diameter, prop pitch, number of blades, air density and RPM. Prop efficiency is a factor too.

For an 18 lbs plane flying scale only you are best running an engine that will turn something like a 20x8 at about 8,000 RPM. An 18x8 turning at only 8,500 RPM will give you about 19-21 lbs thrust and that is just nice for a 18 lbs plane. It will fly nice but you do not have much more in reserve. I think the 20x8 turning at about 8,000 RPM will give you no less than 24-26 lbs thrust. If its scale flying that you want then its okay.

A general idea of engines vs. plane weight (we are taling almost maximum weight for the plane to fly decently and not 3D etc), does not apply for planes like Piper Cubs and 3D or Bipes. I got all this form trial ... its not definitive but a general idea of where to start with. Nothing beats people who have flown the plane before.

46 2C - 6.5 lbs
50 2C - 7 lbs
61 2C - 7.5-8 lbs
91 2C - 8.5-9 lbs
120 2C - 12 lbs

72/82 4C - 7 lbs
91 4C - 8.5 lbs
100 4C - 10 lbs
110 YS - 12 lbs