The 3.5 (61cc) BME is a very strong motor for its displacement, turning 22" props with ease, and the 24-8 Menz way faster than most people would believe. In an 18 lbs plane it will fly well, but how well YOU perceive it to perform is the question. Up to 20 lbs the 61 will have good authority and be able to pull out of a TR very well. After that it will become marginal, at best.

Personally, for REALLY rip-roaring performance I'd prefer to see the plane a touch lighter, in the 16 lbs range, but at 18 lbs (an HONEST 18 lbs) I think the BME-61 will surprise you. Compare the BME-61 to a Zenoah G-62, the power difference is silly, and the G-62 would fly that plane well.