I agree with MINNFLYER..you will have plenty of pull with a 120 4 stroke on a 8-9 lb plane and my preference is a 4 stroke,I hate the wing-ding-ding--ding sound of a 2c engine either in a motorcycle,lawn mower or anything else.
I love the deep sound of my Harley.

When is the last time you heard that 2c sound in an real airplane except for an ultralight.[sm=lol.gif]
Also you will have no problem with a Magnum 4 stroke,Magnum is a good engine!