Roald,
I think your FS-70 will be more than enought, I put an FS-91 S/P in mine and let me tell you that it is more than enough, Yes, my plane lifts off in no more than 10 meters at full throttle, but I have a lot of stability problems due to the engine´s high torque in flight. Every time I pull full throttle stick, my plane turns right as if I give right ailerons input.
I modified my engine and it gives 300 rpm more than my Saito FA-100 with the same APC 15X4 W prop.
Now, I´m planning to install maybe an O.S. 70 Ultimate on it or maybe an O.S. 55 Ax will do the proper job.

Best regards.