Ok chaps need help on what engine to fit to a seagull spacewalker 40, this will be my second model. Its a large 40 size (more of a 50) at 61 inch wingspan.

I wanted a four stroke for it and after reading and looking about I thought the sc 90fs I have already for a future sig hog bipe would be to heavy for it.
With this in mind I bought aasp 61 four stroke as it weighs about the same as a 46 size two sroke and a 52 four stroke.
I looked at the sc 70 fs but did not buy it as itweighsmuch the sameas thesc90fs that I have (prob would have been the perfect choice).

Anyway turns out when balanced upside down at 7.5 to 8 cm from leading edge the plane will balance perfectly with no weight added with the 91fs.

To balance with the 61 fs I will need to add weight to the nose even if I move the battery to the firewall, this plane seems to be very tail heavy and able to take a large engine.

Im worried that the plane may be underpowered with the 61 fs and overpowered with the 91 fs, another engine is prob the real answer but if you had to choose one of the two engines I have what would you do.