The beam mounting of the BME is fine, just use the Great Planes adjustable mount (modified a bit) or the Hanger 9 ultra mount (metal).
You won't be disappointed with either of these two engines. I have a BME-50 on a Great Planes Patty wagstaff Extra @ just under 16lbs. and the power to weight is awesome. I have a DA_50 on order, which will go on my Wild Hare Edge which will come out heavier than the Patty, but reports have been very favorable with this combo. The Wild hare should come out to be 16-1/2 to 17 lbs.

What is your Yak going to weigh.